Fix issue with DST starting/ending causing overlaps/gaps (#5266)

# What this PR does

This older version of recurring_ical_events does not call the pytz
.normalize() function, which can cause some invalid datetime objects to
return when a DST swap happens. For example: Nov 3, 2024 9:00 AM CDT
instead of the correct 8:00 AM CST). By calling tz.normalize on the end
date and checking if the time zone information changed, we can detect
when DST starts/stops and adjust the end date accordingly.

| | DST stopping on November 3, 2024: | DST starting on March 9, 2024 |

|-|-----------------------------------------|-----------------------------------|
| Before |
![image](https://github.com/user-attachments/assets/933bce80-9b6a-475b-88f2-6356d0e3a6fd)
|
![image](https://github.com/user-attachments/assets/264b816f-6f40-4f14-bbc0-1d03f7b74ac4)
| After |
![image](https://github.com/user-attachments/assets/fbd71991-c4f8-4685-a527-6dbb147b2cb6)
|
![image](https://github.com/user-attachments/assets/ccd932df-2ab4-4472-bc90-045372712f75)
|

## Which issue(s) this PR closes

Closes #5247

## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [ ] Documentation added (or `pr:no public docs` PR label added if not
required)
- [ ] Added the relevant release notes label (see labels prefixed w/
`release:`). These labels dictate how your PR will
    show up in the autogenerated release notes.

---------

Co-authored-by: Matias Bordese <mbordese@gmail.com>

engine/apps/schedules/ical_events/adapter/amixr_recurring_ical_events_adapter.py

@@ -20,6 +20,15 @@ EXTRA_LOOKUP_DAYS = 16
 
 
 class AmixrRecurringIcalEventsAdapter(IcalService):
+    def _normalize(self, dt: datetime.datetime) -> datetime.datetime:
+        tz = getattr(dt, "tzinfo", None)
+        if tz:
+            normalized = tz.normalize(dt)
+            if normalized.tzinfo != tz:
+                diff = dt.dst() - normalized.dst()
+                dt = normalized + diff
+        return dt
+
     def get_events_from_ical_between(
         self, calendar: Calendar, start_date: datetime.datetime, end_date: datetime.datetime
     ) -> typing.List[Event]:
@@ -38,6 +47,10 @@ class AmixrRecurringIcalEventsAdapter(IcalService):
             end_date + datetime.timedelta(days=EXTRA_LOOKUP_DAYS),
         )
 
+        for event in events:
+            # account for timezones not being properly calculated when DST changes.
+            event[ICAL_DATETIME_END].dt = self._normalize(event[ICAL_DATETIME_END].dt)
+
         def filter_extra_days(event):
             event_start, event_end = self.get_start_and_end_with_respect_to_event_type(event)
             if event_start > event_end:

engine/apps/schedules/tests/test_on_call_schedule.py

@@ -1818,6 +1818,63 @@ def test_refresh_ical_final_schedule_ok(
                 assert event in expected_events
 
 
+@pytest.mark.django_db
+def test_filter_events_during_dst_change(
+    make_organization,
+    make_user_for_organization,
+    make_schedule,
+    make_on_call_shift,
+):
+    organization = make_organization()
+    u1 = make_user_for_organization(organization)
+
+    schedule = make_schedule(
+        organization,
+        time_zone="America/Chicago",  # UTC-6 or UTC-5 depending on DST
+        schedule_class=OnCallScheduleCalendar,
+    )
+    start_datetime = timezone.datetime(2024, 10, 1, 0, 0, 0, tzinfo=timezone.utc)
+    duration = timezone.timedelta(seconds=60 * 60 * 24 * 7)  # 1 week
+    data = {
+        "start": start_datetime,
+        "rotation_start": start_datetime,
+        "duration": duration,
+        "frequency": CustomOnCallShift.FREQUENCY_WEEKLY,
+        "week_start": CustomOnCallShift.MONDAY,
+    }
+    on_call_shift = make_on_call_shift(
+        organization=organization, shift_type=CustomOnCallShift.TYPE_ROLLING_USERS_EVENT, **data
+    )
+    on_call_shift.add_rolling_users([[u1]])
+    schedule.custom_on_call_shifts.add(on_call_shift)
+
+    schedule.refresh_ical_file()
+
+    # week with DST change
+    start_date = timezone.datetime(2024, 11, 4, 0, 0, 0, tzinfo=timezone.utc)
+    end_date = start_date + timezone.timedelta(days=1)
+    events = schedule.filter_events(start_date, end_date)
+    expected = {
+        "start": timezone.datetime(2024, 10, 29, 5, 0, 0, tzinfo=timezone.utc),
+        "end": timezone.datetime(2024, 11, 5, 6, 0, 0, tzinfo=timezone.utc),
+    }
+    assert len(events) == 1
+    returned = {"start": events[0]["start"], "end": events[0]["end"]}
+    assert returned == expected
+
+    # week with DST change back
+    start_date = timezone.datetime(2025, 3, 10, 0, 0, 0, tzinfo=timezone.utc)
+    end_date = start_date + timezone.timedelta(days=1)
+    events = schedule.filter_events(start_date, end_date)
+    expected = {
+        "start": timezone.datetime(2025, 3, 4, 6, 0, 0, tzinfo=timezone.utc),
+        "end": timezone.datetime(2025, 3, 11, 5, 0, 0, tzinfo=timezone.utc),
+    }
+    assert len(events) == 1
+    returned = {"start": events[0]["start"], "end": events[0]["end"]}
+    assert returned == expected
+
+
 @pytest.mark.django_db
 def test_refresh_ical_final_schedule_cancel_deleted_events(
     make_organization,