| # Local Time |
| |
| Local time is an estimate of the standard time at the device’s location, aka |
| "wall clock time". Local time is derived from UTC and time zone. Local time is |
| more complex and has more failure modes than the other time standards so as a |
| developer you should prefer monotonic or UTC time unless there is an explicit |
| need to use local time. Please refer to [time overview](overview.md) for more |
| information on choosing between time standards and |
| [UTC overview](utc/overview.md) for more information on the implementation and |
| behavior of UTC for Fuchsia. |
| |
| Time zone is maintained as one component of the internationalization settings |
| by [`setui_service`](/src/settings/service/meta/setui_service.cml) (for more information, see |
| [`fuchsia.settings.Intl`](https://fuchsia.dev/reference/fidl/fuchsia.settings#Intl)). |
| [`intl_services`](/src/intl/intl_services/meta/intl.cml) exposes a more |
| convenient |
| [`fuchsia.intl.PropertyProvider`](https://fuchsia.dev/reference/fidl/fuchsia.intl#PropertyProvider) |
| FIDL interface that you may use to read the preferred time zone, read any |
| optional alternate time zones, and receive time zone change events. |
| |
| Low level language runtimes such as C, C++, and Rust do not directly implement |
| local time because that would have required a dependency on |
| `fuchsia.intl.PropertyProvider` and ICU from all components written in these |
| languages. [`localtime`](/zircon/third_party/ulib/musl/src/time/localtime.c) |
| functions in the Fuchsia musl implementation always return a UTC time. If you |
| need to use local time from these languages you should use |
| [`fuchsia.intl.PropertyProvider.GetProfile`](https://fuchsia.dev/reference/fidl/fuchsia.intl#PropertyProvider.GetProfile) |
| to read the preferred timezone, use the |
| [ICU timezone data](/docs/development/internationalization/icu_data.md) to |
| determine the timezone offset, and then apply this offset to the current UTC |
| time. |
| |
| As with UTC, a Fuchsia device cannot produce a valid local time until it has |
| received a valid time from either the network or a real time clock. The time |
| zone is usually set based on user input so you should assume that the time zone |
| may change at any time and that the timezone is potentially inaccurate (i.e. the |
| reported time zone may not match the legally defined civil time at the device's |
| present location). Local time may jump either forwards or backwards, for example |
| at daylight savings time transitions. |
