/* gdatetime.c
 *
 * Copyright (C) 2009-2010 Christian Hergert <chris@dronelabs.com>
 * Copyright (C) 2010 Thiago Santos <thiago.sousa.santos@collabora.co.uk>
 * Copyright (C) 2010 Emmanuele Bassi <ebassi@linux.intel.com>
 * Copyright © 2010 Codethink Limited
 *
 * This library is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as
 * published by the Free Software Foundation; either version 2.1 of the
 * licence, or (at your option) any later version.
 *
 * This is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
 * License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301
 * USA.
 *
 * Authors: Christian Hergert <chris@dronelabs.com>
 *          Thiago Santos <thiago.sousa.santos@collabora.co.uk>
 *          Emmanuele Bassi <ebassi@linux.intel.com>
 *          Ryan Lortie <desrt@desrt.ca>
 */

/* Algorithms within this file are based on the Calendar FAQ by
 * Claus Tondering.  It can be found at
 * http://www.tondering.dk/claus/cal/calendar29.txt
 *
 * Copyright and disclaimer
 * ------------------------
 *   This document is Copyright (C) 2008 by Claus Tondering.
 *   E-mail: claus@tondering.dk. (Please include the word
 *   "calendar" in the subject line.)
 *   The document may be freely distributed, provided this
 *   copyright notice is included and no money is charged for
 *   the document.
 *
 *   This document is provided "as is". No warranties are made as
 *   to its correctness.
 */

/* Prologue {{{1 */

#include "config.h"

#include <stdlib.h>
#include <string.h>

#ifdef HAVE_LANGINFO_TIME
#include <langinfo.h>
#endif

#include "gdatetime.h"

#include "gslice.h"
#include "gatomic.h"
#include "gcharset.h"
#include "gconvert.h"
#include "gfileutils.h"
#include "ghash.h"
#include "gmain.h"
#include "gmappedfile.h"
#include "gstrfuncs.h"
#include "gtestutils.h"
#include "gthread.h"
#include "gtimezone.h"

#include "glibintl.h"

#ifndef G_OS_WIN32
#include <sys/time.h>
#include <time.h>
#endif /* !G_OS_WIN32 */

/**
 * SECTION:date-time
 * @title: GDateTime
 * @short_description: a structure representing Date and Time
 * @see_also: #GTimeZone
 *
 * #GDateTime is a structure that combines a Gregorian date and time
 * into a single structure.  It provides many conversion and methods to
 * manipulate dates and times.  Time precision is provided down to
 * microseconds and the time can range (proleptically) from 0001-01-01
 * 00:00:00 to 9999-12-31 23:59:59.999999.  #GDateTime follows POSIX
 * time in the sense that it is oblivious to leap seconds.
 *
 * #GDateTime is an immutable object; once it has been created it cannot
 * be modified further.  All modifiers will create a new #GDateTime.
 * Nearly all such functions can fail due to the date or time going out
 * of range, in which case %NULL will be returned.
 *
 * #GDateTime is reference counted: the reference count is increased by calling
 * g_date_time_ref() and decreased by calling g_date_time_unref(). When the
 * reference count drops to 0, the resources allocated by the #GDateTime
 * structure are released.
 *
 * Many parts of the API may produce non-obvious results.  As an
 * example, adding two months to January 31st will yield March 31st
 * whereas adding one month and then one month again will yield either
 * March 28th or March 29th.  Also note that adding 24 hours is not
 * always the same as adding one day (since days containing daylight
 * savings time transitions are either 23 or 25 hours in length).
 *
 * #GDateTime is available since GLib 2.26.
 */

struct _GDateTime
{
  /* Microsecond timekeeping within Day */
  guint64 usec;

  /* TimeZone information */
  GTimeZone *tz;
  gint interval;

  /* 1 is 0001-01-01 in Proleptic Gregorian */
  gint32 days;

  volatile gint ref_count;
};

/* Time conversion {{{1 */

#define UNIX_EPOCH_START     719163
#define INSTANT_TO_UNIX(instant) \
  ((instant)/USEC_PER_SECOND - UNIX_EPOCH_START * SEC_PER_DAY)
#define UNIX_TO_INSTANT(unix) \
  (((unix) + UNIX_EPOCH_START * SEC_PER_DAY) * USEC_PER_SECOND)

#define DAYS_IN_4YEARS    1461    /* days in 4 years */
#define DAYS_IN_100YEARS  36524   /* days in 100 years */
#define DAYS_IN_400YEARS  146097  /* days in 400 years  */

#define USEC_PER_SECOND      (G_GINT64_CONSTANT (1000000))
#define USEC_PER_MINUTE      (G_GINT64_CONSTANT (60000000))
#define USEC_PER_HOUR        (G_GINT64_CONSTANT (3600000000))
#define USEC_PER_MILLISECOND (G_GINT64_CONSTANT (1000))
#define USEC_PER_DAY         (G_GINT64_CONSTANT (86400000000))
#define SEC_PER_DAY          (G_GINT64_CONSTANT (86400))

#define SECS_PER_MINUTE (60)
#define SECS_PER_HOUR   (60 * SECS_PER_MINUTE)
#define SECS_PER_DAY    (24 * SECS_PER_HOUR)
#define SECS_PER_YEAR   (365 * SECS_PER_DAY)
#define SECS_PER_JULIAN (DAYS_PER_PERIOD * SECS_PER_DAY)

#define GREGORIAN_LEAP(y)    ((((y) % 4) == 0) && (!((((y) % 100) == 0) && (((y) % 400) != 0))))
#define JULIAN_YEAR(d)       ((d)->julian / 365.25)
#define DAYS_PER_PERIOD      (G_GINT64_CONSTANT (2914695))

static const guint16 days_in_months[2][13] =
{
  { 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 },
  { 0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 }
};

static const guint16 days_in_year[2][13] =
{
  {  0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365 },
  {  0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366 }
};

#ifdef HAVE_LANGINFO_TIME

#define GET_AMPM(d) ((g_date_time_get_hour (d) < 12) ? \
                     nl_langinfo (AM_STR) : \
                     nl_langinfo (PM_STR))

#define PREFERRED_DATE_TIME_FMT nl_langinfo (D_T_FMT)
#define PREFERRED_DATE_FMT nl_langinfo (D_FMT)
#define PREFERRED_TIME_FMT nl_langinfo (T_FMT)
#define PREFERRED_TIME_FMT nl_langinfo (T_FMT)
#define PREFERRED_12HR_TIME_FMT nl_langinfo (T_FMT_AMPM)

static const gint weekday_item[2][7] =
{
  { ABDAY_2, ABDAY_3, ABDAY_4, ABDAY_5, ABDAY_6, ABDAY_7, ABDAY_1 },
  { DAY_2, DAY_3, DAY_4, DAY_5, DAY_6, DAY_7, DAY_1 }
};

static const gint month_item[2][12] =
{
  { ABMON_1, ABMON_2, ABMON_3, ABMON_4, ABMON_5, ABMON_6, ABMON_7, ABMON_8, ABMON_9, ABMON_10, ABMON_11, ABMON_12 },
  { MON_1, MON_2, MON_3, MON_4, MON_5, MON_6, MON_7, MON_8, MON_9, MON_10, MON_11, MON_12 },
};

#define WEEKDAY_ABBR(d) nl_langinfo (weekday_item[0][g_date_time_get_day_of_week (d) - 1])
#define WEEKDAY_FULL(d) nl_langinfo (weekday_item[1][g_date_time_get_day_of_week (d) - 1])
#define MONTH_ABBR(d) nl_langinfo (month_item[0][g_date_time_get_month (d) - 1])
#define MONTH_FULL(d) nl_langinfo (month_item[1][g_date_time_get_month (d) - 1])

#else

#define GET_AMPM(d)          ((g_date_time_get_hour (d) < 12)  \
                                       /* Translators: 'before midday' indicator */ \
                                ? C_("GDateTime", "AM") \
                                  /* Translators: 'after midday' indicator */ \
                                : C_("GDateTime", "PM"))

/* Translators: this is the preferred format for expressing the date and the time */
#define PREFERRED_DATE_TIME_FMT C_("GDateTime", "%a %b %e %H:%M:%S %Y")

/* Translators: this is the preferred format for expressing the date */
#define PREFERRED_DATE_FMT C_("GDateTime", "%m/%d/%y")

/* Translators: this is the preferred format for expressing the time */
#define PREFERRED_TIME_FMT C_("GDateTime", "%H:%M:%S")

/* Translators: this is the preferred format for expressing 12 hour time */
#define PREFERRED_12HR_TIME_FMT C_("GDateTime", "%I:%M:%S %p")

#define WEEKDAY_ABBR(d)       (get_weekday_name_abbr (g_date_time_get_day_of_week (d)))
#define WEEKDAY_FULL(d)       (get_weekday_name (g_date_time_get_day_of_week (d)))
#define MONTH_ABBR(d)         (get_month_name_abbr (g_date_time_get_month (d)))
#define MONTH_FULL(d)         (get_month_name (g_date_time_get_month (d)))

static const gchar *
get_month_name (gint month)
{
  switch (month)
    {
    case 1:
      return C_("full month name", "January");
    case 2:
      return C_("full month name", "February");
    case 3:
      return C_("full month name", "March");
    case 4:
      return C_("full month name", "April");
    case 5:
      return C_("full month name", "May");
    case 6:
      return C_("full month name", "June");
    case 7:
      return C_("full month name", "July");
    case 8:
      return C_("full month name", "August");
    case 9:
      return C_("full month name", "September");
    case 10:
      return C_("full month name", "October");
    case 11:
      return C_("full month name", "November");
    case 12:
      return C_("full month name", "December");

    default:
      g_warning ("Invalid month number %d", month);
    }

  return NULL;
}

static const gchar *
get_month_name_abbr (gint month)
{
  switch (month)
    {
    case 1:
      return C_("abbreviated month name", "Jan");
    case 2:
      return C_("abbreviated month name", "Feb");
    case 3:
      return C_("abbreviated month name", "Mar");
    case 4:
      return C_("abbreviated month name", "Apr");
    case 5:
      return C_("abbreviated month name", "May");
    case 6:
      return C_("abbreviated month name", "Jun");
    case 7:
      return C_("abbreviated month name", "Jul");
    case 8:
      return C_("abbreviated month name", "Aug");
    case 9:
      return C_("abbreviated month name", "Sep");
    case 10:
      return C_("abbreviated month name", "Oct");
    case 11:
      return C_("abbreviated month name", "Nov");
    case 12:
      return C_("abbreviated month name", "Dec");

    default:
      g_warning ("Invalid month number %d", month);
    }

  return NULL;
}

static const gchar *
get_weekday_name (gint day)
{
  switch (day)
    {
    case 1:
      return C_("full weekday name", "Monday");
    case 2:
      return C_("full weekday name", "Tuesday");
    case 3:
      return C_("full weekday name", "Wednesday");
    case 4:
      return C_("full weekday name", "Thursday");
    case 5:
      return C_("full weekday name", "Friday");
    case 6:
      return C_("full weekday name", "Saturday");
    case 7:
      return C_("full weekday name", "Sunday");

    default:
      g_warning ("Invalid week day number %d", day);
    }

  return NULL;
}

static const gchar *
get_weekday_name_abbr (gint day)
{
  switch (day)
    {
    case 1:
      return C_("abbreviated weekday name", "Mon");
    case 2:
      return C_("abbreviated weekday name", "Tue");
    case 3:
      return C_("abbreviated weekday name", "Wed");
    case 4:
      return C_("abbreviated weekday name", "Thu");
    case 5:
      return C_("abbreviated weekday name", "Fri");
    case 6:
      return C_("abbreviated weekday name", "Sat");
    case 7:
      return C_("abbreviated weekday name", "Sun");

    default:
      g_warning ("Invalid week day number %d", day);
    }

  return NULL;
}

#endif  /* HAVE_LANGINFO_TIME */

static inline gint
ymd_to_days (gint year,
             gint month,
             gint day)
{
  gint64 days;

  days = (year - 1) * 365 + ((year - 1) / 4) - ((year - 1) / 100)
      + ((year - 1) / 400);

  days += days_in_year[0][month - 1];
  if (GREGORIAN_LEAP (year) && month > 2)
    day++;

  days += day;

  return days;
}

static void
g_date_time_get_week_number (GDateTime *datetime,
                             gint      *week_number,
                             gint      *day_of_week,
                             gint      *day_of_year)
{
  gint a, b, c, d, e, f, g, n, s, month, day, year;

  g_date_time_get_ymd (datetime, &year, &month, &day);

  if (month <= 2)
    {
      a = g_date_time_get_year (datetime) - 1;
      b = (a / 4) - (a / 100) + (a / 400);
      c = ((a - 1) / 4) - ((a - 1) / 100) + ((a - 1) / 400);
      s = b - c;
      e = 0;
      f = day - 1 + (31 * (month - 1));
    }
  else
    {
      a = year;
      b = (a / 4) - (a / 100) + (a / 400);
      c = ((a - 1) / 4) - ((a - 1) / 100) + ((a - 1) / 400);
      s = b - c;
      e = s + 1;
      f = day + (((153 * (month - 3)) + 2) / 5) + 58 + s;
    }

  g = (a + b) % 7;
  d = (f + g - e) % 7;
  n = f + 3 - d;

  if (week_number)
    {
      if (n < 0)
        *week_number = 53 - ((g - s) / 5);
      else if (n > 364 + s)
        *week_number = 1;
      else
        *week_number = (n / 7) + 1;
    }

  if (day_of_week)
    *day_of_week = d + 1;

  if (day_of_year)
    *day_of_year = f + 1;
}

/* Lifecycle {{{1 */

static GDateTime *
g_date_time_alloc (GTimeZone *tz)
{
  GDateTime *datetime;

  datetime = g_slice_new0 (GDateTime);
  datetime->tz = g_time_zone_ref (tz);
  datetime->ref_count = 1;

  return datetime;
}

/**
 * g_date_time_ref:
 * @datetime: a #GDateTime
 *
 * Atomically increments the reference count of @datetime by one.
 *
 * Returns: the #GDateTime with the reference count increased
 *
 * Since: 2.26
 */
GDateTime *
g_date_time_ref (GDateTime *datetime)
{
  g_return_val_if_fail (datetime != NULL, NULL);
  g_return_val_if_fail (datetime->ref_count > 0, NULL);

  g_atomic_int_inc (&datetime->ref_count);

  return datetime;
}

/**
 * g_date_time_unref:
 * @datetime: a #GDateTime
 *
 * Atomically decrements the reference count of @datetime by one.
 *
 * When the reference count reaches zero, the resources allocated by
 * @datetime are freed
 *
 * Since: 2.26
 */
void
g_date_time_unref (GDateTime *datetime)
{
  g_return_if_fail (datetime != NULL);
  g_return_if_fail (datetime->ref_count > 0);

  if (g_atomic_int_dec_and_test (&datetime->ref_count))
    {
      g_time_zone_unref (datetime->tz);
      g_slice_free (GDateTime, datetime);
    }
}

/* Internal state transformers {{{1 */
/*< internal >
 * g_date_time_to_instant:
 * @datetime: a #GDateTime
 *
 * Convert a @datetime into an instant.
 *
 * An instant is a number that uniquely describes a particular
 * microsecond in time, taking time zone considerations into account.
 * (ie: "03:00 -0400" is the same instant as "02:00 -0500").
 *
 * An instant is always positive but we use a signed return value to
 * avoid troubles with C.
 */
static gint64
g_date_time_to_instant (GDateTime *datetime)
{
  gint64 offset;

  offset = g_time_zone_get_offset (datetime->tz, datetime->interval);
  offset *= USEC_PER_SECOND;

  return datetime->days * USEC_PER_DAY + datetime->usec - offset;
}

/*< internal >
 * g_date_time_from_instant:
 * @tz: a #GTimeZone
 * @instant: a instant in time
 *
 * Creates a #GDateTime from a time zone and an instant.
 *
 * This might fail if the time ends up being out of range.
 */
static GDateTime *
g_date_time_from_instant (GTimeZone *tz,
                          gint64     instant)
{
  GDateTime *datetime;
  gint64 offset;

  if (instant < 0 || instant > G_GINT64_CONSTANT (1000000000000000000))
    return NULL;

  datetime = g_date_time_alloc (tz);
  datetime->interval = g_time_zone_find_interval (tz,
                                                  G_TIME_TYPE_UNIVERSAL,
                                                  INSTANT_TO_UNIX (instant));
  offset = g_time_zone_get_offset (datetime->tz, datetime->interval);
  offset *= USEC_PER_SECOND;

  instant += offset;

  datetime->days = instant / USEC_PER_DAY;
  datetime->usec = instant % USEC_PER_DAY;

  if (datetime->days < 1 || 3652059 < datetime->days)
    {
      g_date_time_unref (datetime);
      datetime = NULL;
    }

  return datetime;
}


/*< internal >
 * g_date_time_deal_with_date_change:
 * @datetime: a #GDateTime
 *
 * This function should be called whenever the date changes by adding
 * days, months or years.  It does three things.
 *
 * First, we ensure that the date falls between 0001-01-01 and
 * 9999-12-31 and return %FALSE if it does not.
 *
 * Next we update the ->interval field.
 *
 * Finally, we ensure that the resulting date and time pair exists (by
 * ensuring that our time zone has an interval containing it) and
 * adjusting as required.  For example, if we have the time 02:30:00 on
 * March 13 2010 in Toronto and we add 1 day to it, we would end up with
 * 2:30am on March 14th, which doesn't exist.  In that case, we bump the
 * time up to 3:00am.
 */
static gboolean
g_date_time_deal_with_date_change (GDateTime *datetime)
{
  GTimeType was_dst;
  gint64 full_time;
  gint64 usec;

  if (datetime->days < 1 || datetime->days > 3652059)
    return FALSE;

  was_dst = g_time_zone_is_dst (datetime->tz, datetime->interval);

  full_time = datetime->days * USEC_PER_DAY + datetime->usec;


  usec = full_time % USEC_PER_SECOND;
  full_time /= USEC_PER_SECOND;
  full_time -= UNIX_EPOCH_START * SEC_PER_DAY;

  datetime->interval = g_time_zone_adjust_time (datetime->tz,
                                                was_dst,
                                                &full_time);
  full_time += UNIX_EPOCH_START * SEC_PER_DAY;
  full_time *= USEC_PER_SECOND;
  full_time += usec;

  datetime->days = full_time / USEC_PER_DAY;
  datetime->usec = full_time % USEC_PER_DAY;

  /* maybe daylight time caused us to shift to a different day,
   * but it definitely didn't push us into a different year */
  return TRUE;
}

static GDateTime *
g_date_time_replace_days (GDateTime *datetime,
                          gint       days)
{
  GDateTime *new;

  new = g_date_time_alloc (datetime->tz);
  new->interval = datetime->interval;
  new->usec = datetime->usec;
  new->days = days;

  if (!g_date_time_deal_with_date_change (new))
    {
      g_date_time_unref (new);
      new = NULL;
    }

  return new;
}

/* now/unix/timeval Constructors {{{1 */

/*< internal >
 * g_date_time_new_from_timeval:
 * @tz: a #GTimeZone
 * @tv: a #GTimeVal
 *
 * Creates a #GDateTime corresponding to the given #GTimeVal @tv in the
 * given time zone @tz.
 *
 * The time contained in a #GTimeVal is always stored in the form of
 * seconds elapsed since 1970-01-01 00:00:00 UTC, regardless of the
 * given time zone.
 *
 * This call can fail (returning %NULL) if @tv represents a time outside
 * of the supported range of #GDateTime.
 *
 * You should release the return value by calling g_date_time_unref()
 * when you are done with it.
 *
 * Returns: a new #GDateTime, or %NULL
 *
 * Since: 2.26
 **/
static GDateTime *
g_date_time_new_from_timeval (GTimeZone      *tz,
                              const GTimeVal *tv)
{
  return g_date_time_from_instant (tz, tv->tv_usec +
                                   UNIX_TO_INSTANT (tv->tv_sec));
}

/*< internal >
 * g_date_time_new_from_unix:
 * @tz: a #GTimeZone
 * @t: the Unix time
 *
 * Creates a #GDateTime corresponding to the given Unix time @t in the
 * given time zone @tz.
 *
 * Unix time is the number of seconds that have elapsed since 1970-01-01
 * 00:00:00 UTC, regardless of the time zone given.
 *
 * This call can fail (returning %NULL) if @t represents a time outside
 * of the supported range of #GDateTime.
 *
 * You should release the return value by calling g_date_time_unref()
 * when you are done with it.
 *
 * Returns: a new #GDateTime, or %NULL
 *
 * Since: 2.26
 **/
static GDateTime *
g_date_time_new_from_unix (GTimeZone *tz,
                           gint64     secs)
{
  return g_date_time_from_instant (tz, UNIX_TO_INSTANT (secs));
}

/**
 * g_date_time_new_now:
 * @tz: a #GTimeZone
 *
 * Creates a #GDateTime corresponding to this exact instant in the given
 * time zone @tz.  The time is as accurate as the system allows, to a
 * maximum accuracy of 1 microsecond.
 *
 * This function will always succeed unless the system clock is set to
 * truly insane values (or unless GLib is still being used after the
 * year 9999).
 *
 * You should release the return value by calling g_date_time_unref()
 * when you are done with it.
 *
 * Returns: a new #GDateTime, or %NULL
 *
 * Since: 2.26
 **/
GDateTime *
g_date_time_new_now (GTimeZone *tz)
{
  GTimeVal tv;

  g_get_current_time (&tv);

  return g_date_time_new_from_timeval (tz, &tv);
}

/**
 * g_date_time_new_now_local:
 *
 * Creates a #GDateTime corresponding to this exact instant in the local
 * time zone.
 *
 * This is equivalent to calling g_date_time_new_now() with the time
 * zone returned by g_time_zone_new_local().
 *
 * Returns: a new #GDateTime, or %NULL
 *
 * Since: 2.26
 **/
GDateTime *
g_date_time_new_now_local (void)
{
  GDateTime *datetime;
  GTimeZone *local;

  local = g_time_zone_new_local ();
  datetime = g_date_time_new_now (local);
  g_time_zone_unref (local);

  return datetime;
}

/**
 * g_date_time_new_now_utc:
 *
 * Creates a #GDateTime corresponding to this exact instant in UTC.
 *
 * This is equivalent to calling g_date_time_new_now() with the time
 * zone returned by g_time_zone_new_utc().
 *
 * Returns: a new #GDateTime, or %NULL
 *
 * Since: 2.26
 **/
GDateTime *
g_date_time_new_now_utc (void)
{
  GDateTime *datetime;
  GTimeZone *utc;

  utc = g_time_zone_new_utc ();
  datetime = g_date_time_new_now (utc);
  g_time_zone_unref (utc);

  return datetime;
}

/**
 * g_date_time_new_from_unix_local:
 * @t: the Unix time
 *
 * Creates a #GDateTime corresponding to the given Unix time @t in the
 * local time zone.
 *
 * Unix time is the number of seconds that have elapsed since 1970-01-01
 * 00:00:00 UTC, regardless of the local time offset.
 *
 * This call can fail (returning %NULL) if @t represents a time outside
 * of the supported range of #GDateTime.
 *
 * You should release the return value by calling g_date_time_unref()
 * when you are done with it.
 *
 * Returns: a new #GDateTime, or %NULL
 *
 * Since: 2.26
 **/
GDateTime *
g_date_time_new_from_unix_local (gint64 t)
{
  GDateTime *datetime;
  GTimeZone *local;

  local = g_time_zone_new_local ();
  datetime = g_date_time_new_from_unix (local, t);
  g_time_zone_unref (local);

  return datetime;
}

/**
 * g_date_time_new_from_unix_utc:
 * @t: the Unix time
 *
 * Creates a #GDateTime corresponding to the given Unix time @t in UTC.
 *
 * Unix time is the number of seconds that have elapsed since 1970-01-01
 * 00:00:00 UTC.
 *
 * This call can fail (returning %NULL) if @t represents a time outside
 * of the supported range of #GDateTime.
 *
 * You should release the return value by calling g_date_time_unref()
 * when you are done with it.
 *
 * Returns: a new #GDateTime, or %NULL
 *
 * Since: 2.26
 **/
GDateTime *
g_date_time_new_from_unix_utc (gint64 t)
{
  GDateTime *datetime;
  GTimeZone *utc;

  utc = g_time_zone_new_utc ();
  datetime = g_date_time_new_from_unix (utc, t);
  g_time_zone_unref (utc);

  return datetime;
}

/**
 * g_date_time_new_from_timeval_local:
 * @tv: a #GTimeVal
 *
 * Creates a #GDateTime corresponding to the given #GTimeVal @tv in the
 * local time zone.
 *
 * The time contained in a #GTimeVal is always stored in the form of
 * seconds elapsed since 1970-01-01 00:00:00 UTC, regardless of the
 * local time offset.
 *
 * This call can fail (returning %NULL) if @tv represents a time outside
 * of the supported range of #GDateTime.
 *
 * You should release the return value by calling g_date_time_unref()
 * when you are done with it.
 *
 * Returns: a new #GDateTime, or %NULL
 *
 * Since: 2.26
 **/
GDateTime *
g_date_time_new_from_timeval_local (const GTimeVal *tv)
{
  GDateTime *datetime;
  GTimeZone *local;

  local = g_time_zone_new_local ();
  datetime = g_date_time_new_from_timeval (local, tv);
  g_time_zone_unref (local);

  return datetime;
}

/**
 * g_date_time_new_from_timeval_utc:
 * @tv: a #GTimeVal
 *
 * Creates a #GDateTime corresponding to the given #GTimeVal @tv in UTC.
 *
 * The time contained in a #GTimeVal is always stored in the form of
 * seconds elapsed since 1970-01-01 00:00:00 UTC.
 *
 * This call can fail (returning %NULL) if @tv represents a time outside
 * of the supported range of #GDateTime.
 *
 * You should release the return value by calling g_date_time_unref()
 * when you are done with it.
 *
 * Returns: a new #GDateTime, or %NULL
 *
 * Since: 2.26
 **/
GDateTime *
g_date_time_new_from_timeval_utc (const GTimeVal *tv)
{
  GDateTime *datetime;
  GTimeZone *utc;

  utc = g_time_zone_new_utc ();
  datetime = g_date_time_new_from_timeval (utc, tv);
  g_time_zone_unref (utc);

  return datetime;
}

/* full new functions {{{1 */

/**
 * g_date_time_new:
 * @tz: a #GTimeZone
 * @year: the year component of the date
 * @month: the month component of the date
 * @day: the day component of the date
 * @hour: the hour component of the date
 * @minute: the minute component of the date
 * @seconds: the number of seconds past the minute
 *
 * Creates a new #GDateTime corresponding to the given date and time in
 * the time zone @tz.
 *
 * The @year must be between 1 and 9999, @month between 1 and 12 and @day
 * between 1 and 28, 29, 30 or 31 depending on the month and the year.
 *
 * @hour must be between 0 and 23 and @minute must be between 0 and 59.
 *
 * @seconds must be at least 0.0 and must be strictly less than 60.0.
 * It will be rounded down to the nearest microsecond.
 *
 * If the given time is not representable in the given time zone (for
 * example, 02:30 on March 14th 2010 in Toronto, due to daylight savings
 * time) then the time will be rounded up to the nearest existing time
 * (in this case, 03:00).  If this matters to you then you should verify
 * the return value for containing the same as the numbers you gave.
 *
 * In the case that the given time is ambiguous in the given time zone
 * (for example, 01:30 on November 7th 2010 in Toronto, due to daylight
 * savings time) then the time falling within standard (ie:
 * non-daylight) time is taken.
 *
 * It not considered a programmer error for the values to this function
 * to be out of range, but in the case that they are, the function will
 * return %NULL.
 *
 * You should release the return value by calling g_date_time_unref()
 * when you are done with it.
 *
 * Returns: a new #GDateTime, or %NULL
 *
 * Since: 2.26
 **/
GDateTime *
g_date_time_new (GTimeZone *tz,
                 gint       year,
                 gint       month,
                 gint       day,
                 gint       hour,
                 gint       minute,
                 gdouble    seconds)
{
  GDateTime *datetime;
  gint64 full_time;

  g_return_val_if_fail (tz != NULL, NULL);

  if (year < 1 || year > 9999 ||
      month < 1 || month > 12 ||
      day < 1 || day > 31 ||
      hour < 0 || hour > 23 ||
      minute < 0 || minute > 59 ||
      seconds < 0.0 || seconds >= 60.0)
    return NULL;

  datetime = g_date_time_alloc (tz);
  datetime->days = ymd_to_days (year, month, day);
  datetime->usec = (hour   * USEC_PER_HOUR)
                 + (minute * USEC_PER_MINUTE)
                 + (gint64) (seconds * USEC_PER_SECOND);

  full_time = SEC_PER_DAY *
                (ymd_to_days (year, month, day) - UNIX_EPOCH_START) +
              SECS_PER_HOUR * hour +
              SECS_PER_MINUTE * minute +
              (int) seconds;

  datetime->interval = g_time_zone_adjust_time (datetime->tz,
                                                G_TIME_TYPE_STANDARD,
                                                &full_time);

  full_time += UNIX_EPOCH_START * SEC_PER_DAY;
  datetime->days = full_time / SEC_PER_DAY;
  datetime->usec = (full_time % SEC_PER_DAY) * USEC_PER_SECOND;
  datetime->usec += ((int) (seconds * USEC_PER_SECOND)) % USEC_PER_SECOND;

  return datetime;
}

/**
 * g_date_time_new_local:
 * @year: the year component of the date
 * @month: the month component of the date
 * @day: the day component of the date
 * @hour: the hour component of the date
 * @minute: the minute component of the date
 * @seconds: the number of seconds past the minute
 *
 * Creates a new #GDateTime corresponding to the given date and time in
 * the local time zone.
 *
 * This call is equivalent to calling g_date_time_new() with the time
 * zone returned by g_time_zone_new_local().
 *
 * Returns: a #GDateTime, or %NULL
 *
 * Since: 2.26
 **/
GDateTime *
g_date_time_new_local (gint    year,
                       gint    month,
                       gint    day,
                       gint    hour,
                       gint    minute,
                       gdouble seconds)
{
  GDateTime *datetime;
  GTimeZone *local;

  local = g_time_zone_new_local ();
  datetime = g_date_time_new (local, year, month, day, hour, minute, seconds);
  g_time_zone_unref (local);

  return datetime;
}

/**
 * g_date_time_new_utc:
 * @year: the year component of the date
 * @month: the month component of the date
 * @day: the day component of the date
 * @hour: the hour component of the date
 * @minute: the minute component of the date
 * @seconds: the number of seconds past the minute
 *
 * Creates a new #GDateTime corresponding to the given date and time in
 * UTC.
 *
 * This call is equivalent to calling g_date_time_new() with the time
 * zone returned by g_time_zone_new_utc().
 *
 * Returns: a #GDateTime, or %NULL
 *
 * Since: 2.26
 **/
GDateTime *
g_date_time_new_utc (gint    year,
                     gint    month,
                     gint    day,
                     gint    hour,
                     gint    minute,
                     gdouble seconds)
{
  GDateTime *datetime;
  GTimeZone *utc;

  utc = g_time_zone_new_utc ();
  datetime = g_date_time_new (utc, year, month, day, hour, minute, seconds);
  g_time_zone_unref (utc);

  return datetime;
}

/* Adders {{{1 */

/**
 * g_date_time_add:
 * @datetime: a #GDateTime
 * @timespan: a #GTimeSpan
 *
 * Creates a copy of @datetime and adds the specified timespan to the copy.
 *
 * Returns: the newly created #GDateTime which should be freed with
 *   g_date_time_unref().
 *
 * Since: 2.26
 */
GDateTime*
g_date_time_add (GDateTime *datetime,
                 GTimeSpan  timespan)
{
  return g_date_time_from_instant (datetime->tz, timespan +
                                   g_date_time_to_instant (datetime));
}

/**
 * g_date_time_add_years:
 * @datetime: a #GDateTime
 * @years: the number of years
 *
 * Creates a copy of @datetime and adds the specified number of years to the
 * copy. Add negative values to subtract years.
 *
 * Returns: the newly created #GDateTime which should be freed with
 *   g_date_time_unref().
 *
 * Since: 2.26
 */
GDateTime *
g_date_time_add_years (GDateTime *datetime,
                       gint       years)
{
  gint year, month, day;

  g_return_val_if_fail (datetime != NULL, NULL);

  if (years < -10000 || years > 10000)
    return NULL;

  g_date_time_get_ymd (datetime, &year, &month, &day);
  year += years;

  /* only possible issue is if we've entered a year with no February 29
   */
  if (month == 2 && day == 29 && !GREGORIAN_LEAP (year))
    day = 28;

  return g_date_time_replace_days (datetime, ymd_to_days (year, month, day));
}

/**
 * g_date_time_add_months:
 * @datetime: a #GDateTime
 * @months: the number of months
 *
 * Creates a copy of @datetime and adds the specified number of months to the
 * copy. Add negative values to subtract months.
 *
 * Returns: the newly created #GDateTime which should be freed with
 *   g_date_time_unref().
 *
 * Since: 2.26
 */
GDateTime*
g_date_time_add_months (GDateTime *datetime,
                        gint       months)
{
  gint year, month, day;

  g_return_val_if_fail (datetime != NULL, NULL);
  g_date_time_get_ymd (datetime, &year, &month, &day);

  if (months < -120000 || months > 120000)
    return NULL;

  year += months / 12;
  month += months % 12;
  if (month < 1)
    {
      month += 12;
      year--;
    }
  else if (month > 12)
    {
      month -= 12;
      year++;
    }

  day = MIN (day, days_in_months[GREGORIAN_LEAP (year)][month]);

  return g_date_time_replace_days (datetime, ymd_to_days (year, month, day));
}

/**
 * g_date_time_add_weeks:
 * @datetime: a #GDateTime
 * @weeks: the number of weeks
 *
 * Creates a copy of @datetime and adds the specified number of weeks to the
 * copy. Add negative values to subtract weeks.
 *
 * Returns: the newly created #GDateTime which should be freed with
 *   g_date_time_unref().
 *
 * Since: 2.26
 */
GDateTime*
g_date_time_add_weeks (GDateTime *datetime,
                       gint             weeks)
{
  g_return_val_if_fail (datetime != NULL, NULL);

  return g_date_time_add_days (datetime, weeks * 7);
}

/**
 * g_date_time_add_days:
 * @datetime: a #GDateTime
 * @days: the number of days
 *
 * Creates a copy of @datetime and adds the specified number of days to the
 * copy. Add negative values to subtract days.
 *
 * Returns: the newly created #GDateTime which should be freed with
 *   g_date_time_unref().
 *
 * Since: 2.26
 */
GDateTime*
g_date_time_add_days (GDateTime *datetime,
                      gint       days)
{
  g_return_val_if_fail (datetime != NULL, NULL);

  if (days < -3660000 || days > 3660000)
    return NULL;

  return g_date_time_replace_days (datetime, datetime->days + days);
}

/**
 * g_date_time_add_hours:
 * @datetime: a #GDateTime
 * @hours: the number of hours to add
 *
 * Creates a copy of @datetime and adds the specified number of hours.
 * Add negative values to subtract hours.
 *
 * Returns: the newly created #GDateTime which should be freed with
 *   g_date_time_unref().
 *
 * Since: 2.26
 */
GDateTime*
g_date_time_add_hours (GDateTime *datetime,
                       gint       hours)
{
  return g_date_time_add (datetime, hours * USEC_PER_HOUR);
}

/**
 * g_date_time_add_minutes:
 * @datetime: a #GDateTime
 * @minutes: the number of minutes to add
 *
 * Creates a copy of @datetime adding the specified number of minutes.
 * Add negative values to subtract minutes.
 *
 * Returns: the newly created #GDateTime which should be freed with
 *   g_date_time_unref().
 *
 * Since: 2.26
 */
GDateTime*
g_date_time_add_minutes (GDateTime *datetime,
                         gint             minutes)
{
  return g_date_time_add (datetime, minutes * USEC_PER_MINUTE);
}


/**
 * g_date_time_add_seconds:
 * @datetime: a #GDateTime
 * @seconds: the number of seconds to add
 *
 * Creates a copy of @datetime and adds the specified number of seconds.
 * Add negative values to subtract seconds.
 *
 * Returns: the newly created #GDateTime which should be freed with
 *   g_date_time_unref().
 *
 * Since: 2.26
 */
GDateTime*
g_date_time_add_seconds (GDateTime *datetime,
                         gdouble    seconds)
{
  return g_date_time_add (datetime, seconds * USEC_PER_SECOND);
}

/**
 * g_date_time_add_full:
 * @datetime: a #GDateTime
 * @years: the number of years to add
 * @months: the number of months to add
 * @days: the number of days to add
 * @hours: the number of hours to add
 * @minutes: the number of minutes to add
 * @seconds: the number of seconds to add
 *
 * Creates a new #GDateTime adding the specified values to the current date and
 * time in @datetime. Add negative values to subtract.
 *
 * Returns: the newly created #GDateTime that should be freed with
 *   g_date_time_unref().
 *
 * Since: 2.26
 */
GDateTime *
g_date_time_add_full (GDateTime *datetime,
                      gint       years,
                      gint       months,
                      gint       days,
                      gint       hours,
                      gint       minutes,
                      gdouble    seconds)
{
  gint year, month, day;
  gint64 full_time;
  GDateTime *new;
  gint interval;

  g_return_val_if_fail (datetime != NULL, NULL);
  g_date_time_get_ymd (datetime, &year, &month, &day);

  months += years * 12;

  if (months < -120000 || months > 120000)
    return NULL;

  if (days < -3660000 || days > 3660000)
    return NULL;

  year += months / 12;
  month += months % 12;
  if (month < 1)
    {
      month += 12;
      year--;
    }
  else if (month > 12)
    {
      month -= 12;
      year++;
    }

  day = MIN (day, days_in_months[GREGORIAN_LEAP (year)][month]);

  /* full_time is now in unix (local) time */
  full_time = datetime->usec / USEC_PER_SECOND + SEC_PER_DAY *
    (ymd_to_days (year, month, day) + days - UNIX_EPOCH_START);

  interval = g_time_zone_adjust_time (datetime->tz,
                                      g_time_zone_is_dst (datetime->tz,
                                                          datetime->interval),
                                      &full_time);

  /* move to UTC unix time */
  full_time -= g_time_zone_get_offset (datetime->tz, interval);

  /* convert back to an instant, add back fractional seconds */
  full_time += UNIX_EPOCH_START * SEC_PER_DAY;
  full_time = full_time * USEC_PER_SECOND +
              datetime->usec % USEC_PER_SECOND;

  /* do the actual addition now */
  full_time += (hours * USEC_PER_HOUR) +
               (minutes * USEC_PER_MINUTE) +
               (gint64) (seconds * USEC_PER_SECOND);

  /* find the new interval */
  interval = g_time_zone_find_interval (datetime->tz,
                                        G_TIME_TYPE_UNIVERSAL,
                                        INSTANT_TO_UNIX (full_time));

  /* convert back into local time */
  full_time += USEC_PER_SECOND *
               g_time_zone_get_offset (datetime->tz, interval);

  /* split into days and usec of a new datetime */
  new = g_date_time_alloc (datetime->tz);
  new->interval = interval;
  new->days = full_time / USEC_PER_DAY;
  new->usec = full_time % USEC_PER_DAY;

  /* XXX validate */

  return new;
}

/* Compare, difference, hash, equal {{{1 */
/**
 * g_date_time_compare:
 * @dt1: first #GDateTime to compare
 * @dt2: second #GDateTime to compare
 *
 * A comparison function for #GDateTimes that is suitable
 * as a #GCompareFunc. Both #GDateTimes must be non-%NULL.
 *
 * Returns: -1, 0 or 1 if @dt1 is less than, equal to or greater
 *   than @dt2.
 *
 * Since: 2.26
 */
gint
g_date_time_compare (gconstpointer dt1,
                     gconstpointer dt2)
{
  gint64 difference;

  difference = g_date_time_difference ((GDateTime *) dt1, (GDateTime *) dt2);

  if (difference < 0)
    return -1;

  else if (difference > 0)
    return 1;

  else
    return 0;
}

/**
 * g_date_time_difference:
 * @end: a #GDateTime
 * @begin: a #GDateTime
 *
 * Calculates the difference in time between @end and @begin.  The
 * #GTimeSpan that is returned is effectively @end - @begin (ie:
 * positive if the first parameter is larger).
 *
 * Returns: the difference between the two #GDateTime, as a time
 *   span expressed in microseconds.
 *
 * Since: 2.26
 */
GTimeSpan
g_date_time_difference (GDateTime *end,
                        GDateTime *begin)
{
  g_return_val_if_fail (begin != NULL, 0);
  g_return_val_if_fail (end != NULL, 0);

  return g_date_time_to_instant (end) -
         g_date_time_to_instant (begin);
}

/**
 * g_date_time_hash:
 * @datetime: a #GDateTime
 *
 * Hashes @datetime into a #guint, suitable for use within #GHashTable.
 *
 * Returns: a #guint containing the hash
 *
 * Since: 2.26
 */
guint
g_date_time_hash (gconstpointer datetime)
{
  return g_date_time_to_instant ((GDateTime *) datetime);
}

/**
 * g_date_time_equal:
 * @dt1: a #GDateTime
 * @dt2: a #GDateTime
 *
 * Checks to see if @dt1 and @dt2 are equal.
 *
 * Equal here means that they represent the same moment after converting
 * them to the same time zone.
 *
 * Returns: %TRUE if @dt1 and @dt2 are equal
 *
 * Since: 2.26
 */
gboolean
g_date_time_equal (gconstpointer dt1,
                   gconstpointer dt2)
{
  return g_date_time_difference ((GDateTime *) dt1, (GDateTime *) dt2) == 0;
}

/* Year, Month, Day Getters {{{1 */
/**
 * g_date_time_get_ymd:
 * @datetime: a #GDateTime.
 * @year: (out) (allow-none): the return location for the gregorian year, or %NULL.
 * @month: (out) (allow-none): the return location for the month of the year, or %NULL.
 * @day: (out) (allow-none): the return location for the day of the month, or %NULL.
 *
 * Retrieves the Gregorian day, month, and year of a given #GDateTime.
 *
 * Since: 2.26
 **/
void
g_date_time_get_ymd (GDateTime *datetime,
                     gint      *year,
                     gint      *month,
                     gint      *day)
{
  gint the_year;
  gint the_month;
  gint the_day;
  gint remaining_days;
  gint y100_cycles;
  gint y4_cycles;
  gint y1_cycles;
  gint preceding;
  gboolean leap;

  g_return_if_fail (datetime != NULL);

  remaining_days = datetime->days;

  /*
   * We need to convert an offset in days to its year/month/day representation.
   * Leap years makes this a little trickier than it should be, so we use
   * 400, 100 and 4 years cycles here to get to the correct year.
   */

  /* Our days offset starts sets 0001-01-01 as day 1, if it was day 0 our
   * math would be simpler, so let's do it */
  remaining_days--;

  the_year = (remaining_days / DAYS_IN_400YEARS) * 400 + 1;
  remaining_days = remaining_days % DAYS_IN_400YEARS;

  y100_cycles = remaining_days / DAYS_IN_100YEARS;
  remaining_days = remaining_days % DAYS_IN_100YEARS;
  the_year += y100_cycles * 100;

  y4_cycles = remaining_days / DAYS_IN_4YEARS;
  remaining_days = remaining_days % DAYS_IN_4YEARS;
  the_year += y4_cycles * 4;

  y1_cycles = remaining_days / 365;
  the_year += y1_cycles;
  remaining_days = remaining_days % 365;

  if (y1_cycles == 4 || y100_cycles == 4) {
    g_assert (remaining_days == 0);

    /* special case that indicates that the date is actually one year before,
     * in the 31th of December */
    the_year--;
    the_month = 12;
    the_day = 31;
    goto end;
  }

  /* now get the month and the day */
  leap = y1_cycles == 3 && (y4_cycles != 24 || y100_cycles == 3);

  g_assert (leap == GREGORIAN_LEAP(the_year));

  the_month = (remaining_days + 50) >> 5;
  preceding = (days_in_year[0][the_month - 1] + (the_month > 2 && leap));
  if (preceding > remaining_days)
    {
      /* estimate is too large */
      the_month -= 1;
      preceding -= leap ? days_in_months[1][the_month]
                        : days_in_months[0][the_month];
    }

  remaining_days -= preceding;
  g_assert(0 <= remaining_days);

  the_day = remaining_days + 1;

end:
  if (year)
    *year = the_year;
  if (month)
    *month = the_month;
  if (day)
    *day = the_day;
}

/**
 * g_date_time_get_year:
 * @datetime: A #GDateTime
 *
 * Retrieves the year represented by @datetime in the Gregorian calendar.
 *
 * Returns: the year represented by @datetime
 *
 * Since: 2.26
 */
gint
g_date_time_get_year (GDateTime *datetime)
{
  gint year;

  g_return_val_if_fail (datetime != NULL, 0);

  g_date_time_get_ymd (datetime, &year, NULL, NULL);

  return year;
}

/**
 * g_date_time_get_month:
 * @datetime: a #GDateTime
 *
 * Retrieves the month of the year represented by @datetime in the Gregorian
 * calendar.
 *
 * Returns: the month represented by @datetime
 *
 * Since: 2.26
 */
gint
g_date_time_get_month (GDateTime *datetime)
{
  gint month;

  g_return_val_if_fail (datetime != NULL, 0);

  g_date_time_get_ymd (datetime, NULL, &month, NULL);

  return month;
}

/**
 * g_date_time_get_day_of_month:
 * @datetime: a #GDateTime
 *
 * Retrieves the day of the month represented by @datetime in the gregorian
 * calendar.
 *
 * Returns: the day of the month
 *
 * Since: 2.26
 */
gint
g_date_time_get_day_of_month (GDateTime *datetime)
{
  gint           day_of_year,
                 i;
  const guint16 *days;
  guint16        last = 0;

  g_return_val_if_fail (datetime != NULL, 0);

  days = days_in_year[GREGORIAN_LEAP (g_date_time_get_year (datetime)) ? 1 : 0];
  g_date_time_get_week_number (datetime, NULL, NULL, &day_of_year);

  for (i = 1; i <= 12; i++)
    {
      if (days [i] >= day_of_year)
        return day_of_year - last;
      last = days [i];
    }

  g_warn_if_reached ();
  return 0;
}

/* Week of year / day of week getters {{{1 */
/**
 * g_date_time_get_week_numbering_year:
 * @datetime: a #GDateTime
 *
 * Returns the ISO 8601 week-numbering year in which the week containing
 * @datetime falls.
 *
 * This function, taken together with g_date_time_get_week_of_year() and
 * g_date_time_get_day_of_week() can be used to determine the full ISO
 * week date on which @datetime falls.
 *
 * This is usually equal to the normal Gregorian year (as returned by
 * g_date_time_get_year()), except as detailed below:
 *
 * For Thursday, the week-numbering year is always equal to the usual
 * calendar year.  For other days, the number is such that every day
 * within a complete week (Monday to Sunday) is contained within the
 * same week-numbering year.
 *
 * For Monday, Tuesday and Wednesday occurring near the end of the year,
 * this may mean that the week-numbering year is one greater than the
 * calendar year (so that these days have the same week-numbering year
 * as the Thursday occurring early in the next year).
 *
 * For Friday, Saturday and Sunday occurring near the start of the year,
 * this may mean that the week-numbering year is one less than the
 * calendar year (so that these days have the same week-numbering year
 * as the Thursday occurring late in the previous year).
 *
 * An equivalent description is that the week-numbering year is equal to
 * the calendar year containing the majority of the days in the current
 * week (Monday to Sunday).
 *
 * Note that January 1 0001 in the proleptic Gregorian calendar is a
 * Monday, so this function never returns 0.
 *
 * Returns: the ISO 8601 week-numbering year for @datetime
 *
 * Since: 2.26
 **/
gint
g_date_time_get_week_numbering_year (GDateTime *datetime)
{
  gint year, month, day, weekday;

  g_date_time_get_ymd (datetime, &year, &month, &day);
  weekday = g_date_time_get_day_of_week (datetime);

  /* January 1, 2, 3 might be in the previous year if they occur after
   * Thursday.
   *
   *   Jan 1:  Friday, Saturday, Sunday    =>  day 1:  weekday 5, 6, 7
   *   Jan 2:  Saturday, Sunday            =>  day 2:  weekday 6, 7
   *   Jan 3:  Sunday                      =>  day 3:  weekday 7
   *
   * So we have a special case if (day - weekday) <= -4
   */
  if (month == 1 && (day - weekday) <= -4)
    return year - 1;

  /* December 29, 30, 31 might be in the next year if they occur before
   * Thursday.
   *
   *   Dec 31: Monday, Tuesday, Wednesday  =>  day 31: weekday 1, 2, 3
   *   Dec 30: Monday, Tuesday             =>  day 30: weekday 1, 2
   *   Dec 29: Monday                      =>  day 29: weekday 1
   *
   * So we have a special case if (day - weekday) >= 28
   */
  else if (month == 12 && (day - weekday) >= 28)
    return year + 1;

  else
    return year;
}

/**
 * g_date_time_get_week_of_year:
 * @datetime: a #GDateTime
 *
 * Returns the ISO 8601 week number for the week containing @datetime.
 * The ISO 8601 week number is the same for every day of the week (from
 * Moday through Sunday).  That can produce some unusual results
 * (described below).
 *
 * The first week of the year is week 1.  This is the week that contains
 * the first Thursday of the year.  Equivalently, this is the first week
 * that has more than 4 of its days falling within the calendar year.
 *
 * The value 0 is never returned by this function.  Days contained
 * within a year but occurring before the first ISO 8601 week of that
 * year are considered as being contained in the last week of the
 * previous year.  Similarly, the final days of a calendar year may be
 * considered as being part of the first ISO 8601 week of the next year
 * if 4 or more days of that week are contained within the new year.
 *
 * Returns: the ISO 8601 week number for @datetime.
 *
 * Since: 2.26
 */
gint
g_date_time_get_week_of_year (GDateTime *datetime)
{
  gint weeknum;

  g_return_val_if_fail (datetime != NULL, 0);

  g_date_time_get_week_number (datetime, &weeknum, NULL, NULL);

  return weeknum;
}

/**
 * g_date_time_get_day_of_week:
 * @datetime: a #GDateTime
 *
 * Retrieves the ISO 8601 day of the week on which @datetime falls (1 is
 * Monday, 2 is Tuesday... 7 is Sunday).
 *
 * Returns: the day of the week
 *
 * Since: 2.26
 */
gint
g_date_time_get_day_of_week (GDateTime *datetime)
{
  g_return_val_if_fail (datetime != NULL, 0);

  return (datetime->days - 1) % 7 + 1;
}

/* Day of year getter {{{1 */
/**
 * g_date_time_get_day_of_year:
 * @datetime: a #GDateTime
 *
 * Retrieves the day of the year represented by @datetime in the Gregorian
 * calendar.
 *
 * Returns: the day of the year
 *
 * Since: 2.26
 */
gint
g_date_time_get_day_of_year (GDateTime *datetime)
{
  gint doy = 0;

  g_return_val_if_fail (datetime != NULL, 0);

  g_date_time_get_week_number (datetime, NULL, NULL, &doy);
  return doy;
}

/* Time component getters {{{1 */

/**
 * g_date_time_get_hour:
 * @datetime: a #GDateTime
 *
 * Retrieves the hour of the day represented by @datetime
 *
 * Returns: the hour of the day
 *
 * Since: 2.26
 */
gint
g_date_time_get_hour (GDateTime *datetime)
{
  g_return_val_if_fail (datetime != NULL, 0);

  return (datetime->usec / USEC_PER_HOUR);
}

/**
 * g_date_time_get_minute:
 * @datetime: a #GDateTime
 *
 * Retrieves the minute of the hour represented by @datetime
 *
 * Returns: the minute of the hour
 *
 * Since: 2.26
 */
gint
g_date_time_get_minute (GDateTime *datetime)
{
  g_return_val_if_fail (datetime != NULL, 0);

  return (datetime->usec % USEC_PER_HOUR) / USEC_PER_MINUTE;
}

/**
 * g_date_time_get_second:
 * @datetime: a #GDateTime
 *
 * Retrieves the second of the minute represented by @datetime
 *
 * Returns: the second represented by @datetime
 *
 * Since: 2.26
 */
gint
g_date_time_get_second (GDateTime *datetime)
{
  g_return_val_if_fail (datetime != NULL, 0);

  return (datetime->usec % USEC_PER_MINUTE) / USEC_PER_SECOND;
}

/**
 * g_date_time_get_microsecond:
 * @datetime: a #GDateTime
 *
 * Retrieves the microsecond of the date represented by @datetime
 *
 * Returns: the microsecond of the second
 *
 * Since: 2.26
 */
gint
g_date_time_get_microsecond (GDateTime *datetime)
{
  g_return_val_if_fail (datetime != NULL, 0);

  return (datetime->usec % USEC_PER_SECOND);
}

/**
 * g_date_time_get_seconds:
 * @datetime: a #GDateTime
 *
 * Retrieves the number of seconds since the start of the last minute,
 * including the fractional part.
 *
 * Returns: the number of seconds
 *
 * Since: 2.26
 **/
gdouble
g_date_time_get_seconds (GDateTime *datetime)
{
  g_return_val_if_fail (datetime != NULL, 0);

  return (datetime->usec % USEC_PER_MINUTE) / 1000000.0;
}

/* Exporters {{{1 */
/**
 * g_date_time_to_unix:
 * @datetime: a #GDateTime
 *
 * Gives the Unix time corresponding to @datetime, rounding down to the
 * nearest second.
 *
 * Unix time is the number of seconds that have elapsed since 1970-01-01
 * 00:00:00 UTC, regardless of the time zone associated with @datetime.
 *
 * Returns: the Unix time corresponding to @datetime
 *
 * Since: 2.26
 **/
gint64
g_date_time_to_unix (GDateTime *datetime)
{
  return INSTANT_TO_UNIX (g_date_time_to_instant (datetime));
}

/**
 * g_date_time_to_timeval:
 * @datetime: a #GDateTime
 * @tv: a #GTimeVal to modify
 *
 * Stores the instant in time that @datetime represents into @tv.
 *
 * The time contained in a #GTimeVal is always stored in the form of
 * seconds elapsed since 1970-01-01 00:00:00 UTC, regardless of the time
 * zone associated with @datetime.
 *
 * On systems where 'long' is 32bit (ie: all 32bit systems and all
 * Windows systems), a #GTimeVal is incapable of storing the entire
 * range of values that #GDateTime is capable of expressing.  On those
 * systems, this function returns %FALSE to indicate that the time is
 * out of range.
 *
 * On systems where 'long' is 64bit, this function never fails.
 *
 * Returns: %TRUE if successful, else %FALSE
 *
 * Since: 2.26
 **/
gboolean
g_date_time_to_timeval (GDateTime *datetime,
                        GTimeVal  *tv)
{
  tv->tv_sec = INSTANT_TO_UNIX (g_date_time_to_instant (datetime));
  tv->tv_usec = datetime->usec % USEC_PER_SECOND;

  return TRUE;
}

/* Timezone queries {{{1 */
/**
 * g_date_time_get_utc_offset:
 * @datetime: a #GDateTime
 *
 * Determines the offset to UTC in effect at the time and in the time
 * zone of @datetime.
 *
 * The offset is the number of microseconds that you add to UTC time to
 * arrive at local time for the time zone (ie: negative numbers for time
 * zones west of GMT, positive numbers for east).
 *
 * If @datetime represents UTC time, then the offset is always zero.
 *
 * Returns: the number of microseconds that should be added to UTC to
 *          get the local time
 *
 * Since: 2.26
 **/
GTimeSpan
g_date_time_get_utc_offset (GDateTime *datetime)
{
  gint offset;

  g_return_val_if_fail (datetime != NULL, 0);

  offset = g_time_zone_get_offset (datetime->tz, datetime->interval);

  return (gint64) offset * USEC_PER_SECOND;
}

/**
 * g_date_time_get_timezone_abbreviation:
 * @datetime: a #GDateTime
 *
 * Determines the time zone abbreviation to be used at the time and in
 * the time zone of @datetime.
 *
 * For example, in Toronto this is currently "EST" during the winter
 * months and "EDT" during the summer months when daylight savings
 * time is in effect.
 *
 * Returns: (transfer none): the time zone abbreviation. The returned
 *          string is owned by the #GDateTime and it should not be
 *          modified or freed
 *
 * Since: 2.26
 **/
const gchar *
g_date_time_get_timezone_abbreviation (GDateTime *datetime)
{
  g_return_val_if_fail (datetime != NULL, NULL);

  return g_time_zone_get_abbreviation (datetime->tz, datetime->interval);
}

/**
 * g_date_time_is_daylight_savings:
 * @datetime: a #GDateTime
 *
 * Determines if daylight savings time is in effect at the time and in
 * the time zone of @datetime.
 *
 * Returns: %TRUE if daylight savings time is in effect
 *
 * Since: 2.26
 **/
gboolean
g_date_time_is_daylight_savings (GDateTime *datetime)
{
  g_return_val_if_fail (datetime != NULL, FALSE);

  return g_time_zone_is_dst (datetime->tz, datetime->interval);
}

/* Timezone convert {{{1 */
/**
 * g_date_time_to_timezone:
 * @datetime: a #GDateTime
 * @tz: the new #GTimeZone
 *
 * Create a new #GDateTime corresponding to the same instant in time as
 * @datetime, but in the time zone @tz.
 *
 * This call can fail in the case that the time goes out of bounds.  For
 * example, converting 0001-01-01 00:00:00 UTC to a time zone west of
 * Greenwich will fail (due to the year 0 being out of range).
 *
 * You should release the return value by calling g_date_time_unref()
 * when you are done with it.
 *
 * Returns: a new #GDateTime, or %NULL
 *
 * Since: 2.26
 **/
GDateTime *
g_date_time_to_timezone (GDateTime *datetime,
                         GTimeZone *tz)
{
  return g_date_time_from_instant (tz, g_date_time_to_instant (datetime));
}

/**
 * g_date_time_to_local:
 * @datetime: a #GDateTime
 *
 * Creates a new #GDateTime corresponding to the same instant in time as
 * @datetime, but in the local time zone.
 *
 * This call is equivalent to calling g_date_time_to_timezone() with the
 * time zone returned by g_time_zone_new_local().
 *
 * Returns: the newly created #GDateTime
 *
 * Since: 2.26
 **/
GDateTime *
g_date_time_to_local (GDateTime *datetime)
{
  GDateTime *new;
  GTimeZone *local;

  local = g_time_zone_new_local ();
  new = g_date_time_to_timezone (datetime, local);
  g_time_zone_unref (local);

  return new;
}

/**
 * g_date_time_to_utc:
 * @datetime: a #GDateTime
 *
 * Creates a new #GDateTime corresponding to the same instant in time as
 * @datetime, but in UTC.
 *
 * This call is equivalent to calling g_date_time_to_timezone() with the
 * time zone returned by g_time_zone_new_utc().
 *
 * Returns: the newly created #GDateTime
 *
 * Since: 2.26
 **/
GDateTime *
g_date_time_to_utc (GDateTime *datetime)
{
  GDateTime *new;
  GTimeZone *utc;

  utc = g_time_zone_new_utc ();
  new = g_date_time_to_timezone (datetime, utc);
  g_time_zone_unref (utc);

  return new;
}

/* Format {{{1 */

static gboolean
format_z (GString *outstr,
          gint     offset,
          guint    colons)
{
  gint hours;
  gint minutes;
  gint seconds;

  hours = offset / 3600;
  minutes = ABS (offset) / 60 % 60;
  seconds = ABS (offset) % 60;

  switch (colons)
    {
    case 0:
      g_string_append_printf (outstr, "%+03d%02d",
                              hours,
                              minutes);
      break;

    case 1:
      g_string_append_printf (outstr, "%+03d:%02d",
                              hours,
                              minutes);
      break;

    case 2:
      g_string_append_printf (outstr, "%+03d:%02d:%02d",
                              hours,
                              minutes,
                              seconds);
      break;

    case 3:
      g_string_append_printf (outstr, "%+03d", hours);

      if (minutes != 0 || seconds != 0)
        {
          g_string_append_printf (outstr, ":%02d", minutes);

          if (seconds != 0)
            g_string_append_printf (outstr, ":%02d", seconds);
        }
      break;

    default:
      return FALSE;
    }

  return TRUE;
}

static void
format_number (GString  *str,
               gboolean  use_alt_digits,
               gchar    *pad,
               gint      width,
               guint32   number)
{
  const gchar *ascii_digits[10] = {
    "0", "1", "2", "3", "4", "5", "6", "7", "8", "9"
  };
  const gchar **digits = ascii_digits;
  const gchar *tmp[10];
  gint i = 0;

  g_return_if_fail (width <= 10);

#ifdef HAVE_LANGINFO_OUTDIGIT
  if (use_alt_digits)
    {
      static const gchar *alt_digits[10];
      static gsize initialised;
      /* 2^32 has 10 digits */

      if G_UNLIKELY (g_once_init_enter (&initialised))
        {
#define DO_DIGIT(n) \
        alt_digits[n] = nl_langinfo (_NL_CTYPE_OUTDIGIT## n ##_MB)
          DO_DIGIT(0); DO_DIGIT(1); DO_DIGIT(2); DO_DIGIT(3); DO_DIGIT(4);
          DO_DIGIT(5); DO_DIGIT(6); DO_DIGIT(7); DO_DIGIT(8); DO_DIGIT(9);
#undef DO_DIGIT
          g_once_init_leave (&initialised, TRUE);
        }

      digits = alt_digits;
    }
#endif /* HAVE_LANGINFO_OUTDIGIT */

  do
    {
      tmp[i++] = digits[number % 10];
      number /= 10;
    }
  while (number);

  while (pad && i < width)
    tmp[i++] = *pad == '0' ? digits[0] : pad;

  /* should really be impossible */
  g_assert (i <= 10);

  while (i)
    g_string_append (str, tmp[--i]);
}

static gboolean g_date_time_format_locale (GDateTime   *datetime,
					   const gchar *format,
					   GString     *outstr,
					   gboolean     locale_is_utf8);

/* g_date_time_format() subroutine that takes a locale-encoded format
 * string and produces a locale-encoded date/time string.
 */
static gboolean
g_date_time_locale_format_locale (GDateTime   *datetime,
				  const gchar *format,
				  GString     *outstr,
				  gboolean     locale_is_utf8)
{
  gchar *utf8_format;
  gboolean success;

  if (locale_is_utf8)
    return g_date_time_format_locale (datetime, format, outstr,
				      locale_is_utf8);

  utf8_format = g_locale_to_utf8 (format, -1, NULL, NULL, NULL);
  if (!utf8_format)
    return FALSE;

  success = g_date_time_format_locale (datetime, utf8_format, outstr,
				       locale_is_utf8);
  g_free (utf8_format);
  return success;
}

/* g_date_time_format() subroutine that takes a UTF-8 format
 * string and produces a locale-encoded date/time string.
 */
static gboolean
g_date_time_format_locale (GDateTime   *datetime,
			   const gchar *format,
			   GString     *outstr,
			   gboolean     locale_is_utf8)
{
  guint     len;
  guint     colons;
  gchar    *tmp;
  gunichar  c;
  gboolean  alt_digits = FALSE;
  gboolean  pad_set = FALSE;
  gchar    *pad = "";
  gchar    *ampm;
  const gchar *tz;

  while (*format)
    {
      len = strcspn (format, "%");
      if (len)
	{
	  if (locale_is_utf8)
	    g_string_append_len (outstr, format, len);
	  else
	    {
	      tmp = g_locale_from_utf8 (format, len, NULL, NULL, NULL);
	      if (!tmp)
		return FALSE;
	      g_string_append (outstr, tmp);
	      g_free (tmp);
	    }
	}

      format += len;
      if (!*format)
	break;

      g_assert (*format == '%');
      format++;
      if (!*format)
	break;

      colons = 0;
      alt_digits = FALSE;
      pad_set = FALSE;

    next_mod:
      c = g_utf8_get_char (format);
      format = g_utf8_next_char (format);
      switch (c)
	{
	case 'a':
	  g_string_append (outstr, WEEKDAY_ABBR (datetime));
	  break;
	case 'A':
	  g_string_append (outstr, WEEKDAY_FULL (datetime));
	  break;
	case 'b':
	  g_string_append (outstr, MONTH_ABBR (datetime));
	  break;
	case 'B':
	  g_string_append (outstr, MONTH_FULL (datetime));
	  break;
	case 'c':
	  {
	    if (!g_date_time_locale_format_locale (datetime, PREFERRED_DATE_TIME_FMT,
						   outstr, locale_is_utf8))
	      return FALSE;
	  }
	  break;
	case 'C':
	  format_number (outstr, alt_digits, pad_set ? pad : "0", 2,
			 g_date_time_get_year (datetime) / 100);
	  break;
	case 'd':
	  format_number (outstr, alt_digits, pad_set ? pad : "0", 2,
			 g_date_time_get_day_of_month (datetime));
	  break;
	case 'e':
	  format_number (outstr, alt_digits, pad_set ? pad : " ", 2,
			 g_date_time_get_day_of_month (datetime));
	  break;
	case 'F':
	  g_string_append_printf (outstr, "%d-%02d-%02d",
				  g_date_time_get_year (datetime),
				  g_date_time_get_month (datetime),
				  g_date_time_get_day_of_month (datetime));
	  break;
	case 'g':
	  format_number (outstr, alt_digits, pad_set ? pad : "0", 2,
			 g_date_time_get_week_numbering_year (datetime) % 100);
	  break;
	case 'G':
	  format_number (outstr, alt_digits, pad_set ? pad : 0, 0,
			 g_date_time_get_week_numbering_year (datetime));
	  break;
	case 'h':
	  g_string_append (outstr, MONTH_ABBR (datetime));
	  break;
	case 'H':
	  format_number (outstr, alt_digits, pad_set ? pad : "0", 2,
			 g_date_time_get_hour (datetime));
	  break;
	case 'I':
	  format_number (outstr, alt_digits, pad_set ? pad : "0", 2,
			 (g_date_time_get_hour (datetime) + 11) % 12 + 1);
	  break;
	case 'j':
	  format_number (outstr, alt_digits, pad_set ? pad : "0", 3,
			 g_date_time_get_day_of_year (datetime));
	  break;
	case 'k':
	  format_number (outstr, alt_digits, pad_set ? pad : " ", 2,
			 g_date_time_get_hour (datetime));
	  break;
	case 'l':
	  format_number (outstr, alt_digits, pad_set ? pad : " ", 2,
			 (g_date_time_get_hour (datetime) + 11) % 12 + 1);
	  break;
	case 'n':
	  g_string_append_c (outstr, '\n');
	  break;
	case 'm':
	  format_number (outstr, alt_digits, pad_set ? pad : "0", 2,
			 g_date_time_get_month (datetime));
	  break;
	case 'M':
	  format_number (outstr, alt_digits, pad_set ? pad : "0", 2,
			 g_date_time_get_minute (datetime));
	  break;
	case 'O':
	  alt_digits = TRUE;
	  goto next_mod;
	case 'p':
	  ampm = (gchar *) GET_AMPM (datetime);
	  if (!locale_is_utf8)
	    {
	      ampm = tmp = g_locale_to_utf8 (ampm, -1, NULL, NULL, NULL);
	      if (!tmp)
		return FALSE;
	    }
	  ampm = g_utf8_strup (ampm, -1);
	  if (!locale_is_utf8)
	    {
	      g_free (tmp);
	      tmp = g_locale_from_utf8 (ampm, -1, NULL, NULL, NULL);
	      g_free (ampm);
	      if (!tmp)
		return FALSE;
	      ampm = tmp;
	    }
	  g_string_append (outstr, ampm);
	  g_free (ampm);
	  break;
	case 'P':
	  ampm = (gchar *) GET_AMPM (datetime);
	  if (!locale_is_utf8)
	    {
	      ampm = tmp = g_locale_to_utf8 (ampm, -1, NULL, NULL, NULL);
	      if (!tmp)
		return FALSE;
	    }
	  ampm = g_utf8_strdown (ampm, -1);
	  if (!locale_is_utf8)
	    {
	      g_free (tmp);
	      tmp = g_locale_from_utf8 (ampm, -1, NULL, NULL, NULL);
	      g_free (ampm);
	      if (!tmp)
		return FALSE;
	      ampm = tmp;
	    }
	  g_string_append (outstr, ampm);
	  g_free (ampm);
	  break;
	case 'r':
	  {
	    if (!g_date_time_locale_format_locale (datetime, PREFERRED_12HR_TIME_FMT,
						   outstr, locale_is_utf8))
	      return FALSE;
	  }
	  break;
	case 'R':
	  g_string_append_printf (outstr, "%02d:%02d",
				  g_date_time_get_hour (datetime),
				  g_date_time_get_minute (datetime));
	  break;
	case 's':
	  g_string_append_printf (outstr, "%" G_GINT64_FORMAT, g_date_time_to_unix (datetime));
	  break;
	case 'S':
	  format_number (outstr, alt_digits, pad_set ? pad : "0", 2,
			 g_date_time_get_second (datetime));
	  break;
	case 't':
	  g_string_append_c (outstr, '\t');
	  break;
	case 'T':
	  g_string_append_printf (outstr, "%02d:%02d:%02d",
				  g_date_time_get_hour (datetime),
				  g_date_time_get_minute (datetime),
				  g_date_time_get_second (datetime));
	  break;
	case 'u':
	  format_number (outstr, alt_digits, 0, 0,
			 g_date_time_get_day_of_week (datetime));
	  break;
	case 'V':
	  format_number (outstr, alt_digits, pad_set ? pad : "0", 2,
			 g_date_time_get_week_of_year (datetime));
	  break;
	case 'w':
	  format_number (outstr, alt_digits, 0, 0,
			 g_date_time_get_day_of_week (datetime) % 7);
	  break;
	case 'x':
	  {
	    if (!g_date_time_locale_format_locale (datetime, PREFERRED_DATE_FMT,
						   outstr, locale_is_utf8))
	      return FALSE;
	  }
	  break;
	case 'X':
	  {
	    if (!g_date_time_locale_format_locale (datetime, PREFERRED_TIME_FMT,
						   outstr, locale_is_utf8))
	      return FALSE;
	  }
	  break;
	case 'y':
	  format_number (outstr, alt_digits, pad_set ? pad : "0", 2,
			 g_date_time_get_year (datetime) % 100);
	  break;
	case 'Y':
	  format_number (outstr, alt_digits, 0, 0,
			 g_date_time_get_year (datetime));
	  break;
	case 'z':
	  {
	    gint64 offset;
	    if (datetime->tz != NULL)
	      offset = g_date_time_get_utc_offset (datetime) / USEC_PER_SECOND;
	    else
	      offset = 0;
	    if (!format_z (outstr, (int) offset, colons))
	      return FALSE;
	  }
	  break;
	case 'Z':
	  tz = g_date_time_get_timezone_abbreviation (datetime);
	  if (!locale_is_utf8)
	    {
	      tz = tmp = g_locale_from_utf8 (tz, -1, NULL, NULL, NULL);
	      if (!tmp)
		return FALSE;
	    }
	  g_string_append (outstr, tz);
	  if (!locale_is_utf8)
	    g_free (tmp);
	  break;
	case '%':
	  g_string_append_c (outstr, '%');
	  break;
	case '-':
	  pad_set = TRUE;
	  pad = "";
	  goto next_mod;
	case '_':
	  pad_set = TRUE;
	  pad = " ";
	  goto next_mod;
	case '0':
	  pad_set = TRUE;
	  pad = "0";
	  goto next_mod;
	case ':':
	  /* Colons are only allowed before 'z' */
	  if (*format && *format != 'z' && *format != ':')
	    return FALSE;
	  colons++;
	  goto next_mod;
	default:
	  return FALSE;
	}
    }

  return TRUE;
}

/**
 * g_date_time_format:
 * @datetime: A #GDateTime
 * @format: a valid UTF-8 string, containing the format for the
 *          #GDateTime
 *
 * Creates a newly allocated string representing the requested @format.
 *
 * The format strings understood by this function are a subset of the
 * strftime() format language as specified by C99.  The \%D, \%U and \%W
 * conversions are not supported, nor is the 'E' modifier.  The GNU
 * extensions \%k, \%l, \%s and \%P are supported, however, as are the
 * '0', '_' and '-' modifiers.
 *
 * In contrast to strftime(), this function always produces a UTF-8
 * string, regardless of the current locale.  Note that the rendering of
 * many formats is locale-dependent and may not match the strftime()
 * output exactly.
 *
 * The following format specifiers are supported:
 *
 * - \%a: the abbreviated weekday name according to the current locale
 * - \%A: the full weekday name according to the current locale
 * - \%b: the abbreviated month name according to the current locale
 * - \%B: the full month name according to the current locale
 * - \%c: the preferred date and time representation for the current locale
 * - \%C: the century number (year/100) as a 2-digit integer (00-99)
 * - \%d: the day of the month as a decimal number (range 01 to 31)
 * - \%e: the day of the month as a decimal number (range  1 to 31)
 * - \%F: equivalent to `%Y-%m-%d` (the ISO 8601 date format)
 * - \%g: the last two digits of the ISO 8601 week-based year as a
 *   decimal number (00-99). This works well with \%V and \%u.
 * - \%G: the ISO 8601 week-based year as a decimal number. This works
 *   well with \%V and \%u.
 * - \%h: equivalent to \%b
 * - \%H: the hour as a decimal number using a 24-hour clock (range 00 to 23)
 * - \%I: the hour as a decimal number using a 12-hour clock (range 01 to 12)
 * - \%j: the day of the year as a decimal number (range 001 to 366)
 * - \%k: the hour (24-hour clock) as a decimal number (range 0 to 23);
 *   single digits are preceded by a blank
 * - \%l: the hour (12-hour clock) as a decimal number (range 1 to 12);
 *   single digits are preceded by a blank
 * - \%m: the month as a decimal number (range 01 to 12)
 * - \%M: the minute as a decimal number (range 00 to 59)
 * - \%p: either "AM" or "PM" according to the given time value, or the
 *   corresponding  strings for the current locale.  Noon is treated as
 *   "PM" and midnight as "AM".
 * - \%P: like \%p but lowercase: "am" or "pm" or a corresponding string for
 *   the current locale
 * - \%r: the time in a.m. or p.m. notation
 * - \%R: the time in 24-hour notation (\%H:\%M)
 * - \%s: the number of seconds since the Epoch, that is, since 1970-01-01
 *   00:00:00 UTC
 * - \%S: the second as a decimal number (range 00 to 60)
 * - \%t: a tab character
 * - \%T: the time in 24-hour notation with seconds (\%H:\%M:\%S)
 * - \%u: the ISO 8601 standard day of the week as a decimal, range 1 to 7,
 *    Monday being 1. This works well with \%G and \%V.
 * - \%V: the ISO 8601 standard week number of the current year as a decimal
 *   number, range 01 to 53, where week 1 is the first week that has at
 *   least 4 days in the new year. See g_date_time_get_week_of_year().
 *   This works well with \%G and \%u.
 * - \%w: the day of the week as a decimal, range 0 to 6, Sunday being 0.
 *   This is not the ISO 8601 standard format -- use \%u instead.
 * - \%x: the preferred date representation for the current locale without
 *   the time
 * - \%X: the preferred time representation for the current locale without
 *   the date
 * - \%y: the year as a decimal number without the century
 * - \%Y: the year as a decimal number including the century
 * - \%z: the time zone as an offset from UTC (+hhmm)
 * - \%:z: the time zone as an offset from UTC (+hh:mm).
 *   This is a gnulib strftime() extension. Since: 2.38
 * - \%::z: the time zone as an offset from UTC (+hh:mm:ss). This is a
 *   gnulib strftime() extension. Since: 2.38
 * - \%:::z: the time zone as an offset from UTC, with : to necessary
 *   precision (e.g., -04, +05:30). This is a gnulib strftime() extension. Since: 2.38
 * - \%Z: the time zone or name or abbreviation
 * - \%\%: a literal \% character
 *
 * Some conversion specifications can be modified by preceding the
 * conversion specifier by one or more modifier characters. The
 * following modifiers are supported for many of the numeric
 * conversions:
 * 
 * - O: Use alternative numeric symbols, if the current locale supports those.
 * - _: Pad a numeric result with spaces. This overrides the default padding
 *   for the specifier.
 * - -: Do not pad a numeric result. This overrides the default padding
 *   for the specifier.
 * - 0: Pad a numeric result with zeros. This overrides the default padding
 *   for the specifier.
 *
 * Returns: a newly allocated string formatted to the requested format
 *     or %NULL in the case that there was an error. The string
 *     should be freed with g_free().
 *
 * Since: 2.26
 */
gchar *
g_date_time_format (GDateTime   *datetime,
                    const gchar *format)
{
  GString  *outstr;
  gchar *utf8;
  gboolean locale_is_utf8 = g_get_charset (NULL);

  g_return_val_if_fail (datetime != NULL, NULL);
  g_return_val_if_fail (format != NULL, NULL);
  g_return_val_if_fail (g_utf8_validate (format, -1, NULL), NULL);

  outstr = g_string_sized_new (strlen (format) * 2);

  if (!g_date_time_format_locale (datetime, format, outstr, locale_is_utf8))
    {
      g_string_free (outstr, TRUE);
      return NULL;
    }

  if (locale_is_utf8)
    return g_string_free (outstr, FALSE);

  utf8 = g_locale_to_utf8 (outstr->str, outstr->len, NULL, NULL, NULL);
  g_string_free (outstr, TRUE);
  return utf8;
}


/* Epilogue {{{1 */
/* vim:set foldmethod=marker: */