diff --git a/src/registry/report_definition.proto b/src/registry/report_definition.proto
index 0839169..0513ced 100644
--- a/src/registry/report_definition.proto
+++ b/src/registry/report_definition.proto
@@ -86,8 +86,8 @@
 
     // For each system_profile SP and each event_vector EV, produces the count
     // of the number of unique devices with system profile SP for which EV
-    // “is accepted” during the aggregation period, which must
-    // be DAYS_1, DAYS_7 or DAYS_30.
+    // “is accepted” during the aggregation period, which must be DAYS_1,
+    // DAYS_7, DAYS_28 or DAYS_30.
     //
     // There are different versions of what “is accepted” means depending on
     // which local aggregation procedure is specified:
@@ -113,7 +113,7 @@
     // Input metric types: OCCURRENCE
     //
     // Local aggregation: AT_LEAST_ONCE, SELECT_FIRST, or SELECT_MOST_COMMON
-    // Local aggregation period: DAYS_1, DAYS_7 or DAYS_30.
+    // Local aggregation period: DAYS_1, DAYS_7, DAYS_28 or DAYS_30.
     // Global aggregation: OCCURRENCE_COUNTS
     //
     // Output report row type: OccurrenceCountReportRow
@@ -152,7 +152,7 @@
     //
     // Local aggregation: COUNT_AS_INTEGER (used with OCCURRENCE metrics) or
     //                    NUMERIC_STAT (used with INTEGER metrics)
-    // Local aggregation period: DAYS_1, DAYS_7 or DAYS_30.
+    // Local aggregation period: DAYS_1, DAYS_7, DAYS_28 or DAYS_30.
     // Global aggregation: INTEGER_HISTOGRAMS
     //
     // Output report row type: IntegerHistogramReportRow
@@ -287,7 +287,7 @@
     //
     // Local aggregation: COUNT_AS_INTEGER (used with OCCURRENCE metrics) or
     //                    NUMERIC_STAT (used with INTEGER metrics)
-    // Local aggregation period: DAYS_1, DAYS_7 or DAYS_30.
+    // Local aggregation period: DAYS_1, DAYS_7, DAYS_28 or DAYS_30.
     // Global aggregation: NUMERIC_STATS
     // System Profile Selection Policy: REPORT_ALL
     //
@@ -360,7 +360,7 @@
     // For each system_profile SP, each event_vector EV, and each string value
     // produces the count of the number of unique devices with system profile
     // SP on which the string value was logged in connection with the EV during
-    // the aggregation period, which must be DAYS_1, DAYS_7 or DAYS_30.
+    // the aggregation period, which must be DAYS_1, DAYS_7, DAYS_28 or DAYS_30.
     //
     // This is similar to the AT_LEAST_ONCE local aggregation procedure for
     // UNIQUE_DEVICE_COUNTS. For example, a report of this type might
@@ -371,7 +371,7 @@
     //
     // Input metric types: STRING
     //
-    // Local aggregation period: DAYS_1, DAYS_7 or DAYS_30.
+    // Local aggregation period: DAYS_1, DAYS_7, DAYS_28 or DAYS_30.
     // Global aggregation: STRING_HISTOGRAMS
     //
     // Output report row type: StringCountReportRow