ref(crons): Improve comment on get_next_schedule (#55399)

src/sentry/monitors/models.py

@@ -78,16 +78,31 @@ class MonitorEnvironmentValidationFailed(Exception):
     pass
 
 
-def get_next_schedule(last_checkin, schedule_type, schedule):
+def get_next_schedule(reference_ts: datetime, schedule_type, schedule):
+    """
+    Given the schedule type and schedule, determine the next timestamp for a
+    schedule from the reference_ts
+
+    Examples:
+
+    >>> get_next_schedule('05:30', ScheduleType.CRONTAB, '0 * * * *')
+    >>> 06:00
+
+    >>> get_next_schedule('05:30', ScheduleType.CRONTAB, '30 * * * *')
+    >>> 06:30
+
+    >>> get_next_schedule('05:35', ScheduleType.INTERVAL, [2, 'hour'])
+    >>> 07:35
+    """
     if schedule_type == ScheduleType.CRONTAB:
-        itr = croniter(schedule, last_checkin)
+        itr = croniter(schedule, reference_ts)
         next_schedule = itr.get_next(datetime)
     elif schedule_type == ScheduleType.INTERVAL:
         interval, unit_name = schedule
         rule = rrule.rrule(
-            freq=SCHEDULE_INTERVAL_MAP[unit_name], interval=interval, dtstart=last_checkin, count=2
+            freq=SCHEDULE_INTERVAL_MAP[unit_name], interval=interval, dtstart=reference_ts, count=2
         )
-        if rule[0] > last_checkin:
+        if rule[0] > reference_ts:
             next_schedule = rule[0]
         else:
             next_schedule = rule[1]