[Rishiii57]

Hey team,

This fixes an AmbiguousTimeError that crashes the contest calendar view when viewing months that start during a daylight saving time fall-back transition at midnight (like /contests/2026/11/ or /contests/2020/11/ for users in the America/Havana timezone).

What was happening

ContestCalendar.get_table() calculates the calendar start and end dates with datetime.datetime.combine(calendar[0][0], time.min) and passes them to make_aware(). In timezones where DST ends right at midnight on Sunday (such as Cuba falling back from Daylight Time to Standard Time on the first Sunday of November), 00:00:00 happens twice. Because make_aware() defaults to is_dst=None, pytz cannot tell which side of the transition to pick and raises pytz.exceptions.AmbiguousTimeError.

What changed

We added a helper method _make_aware() on ContestCalendar that resolves ambiguity by setting is_dst=False when using pytz. This cleanly picks standard time for the midnight boundary during fall-back transitions without crashing. For normal dates or standard zoneinfo, it behaves exactly as before.

Testing

• Verified with a standalone reproducer reproducing the exact crash on unpatched code and confirming clean resolution on patched code.
• Added ContestCalendarTestCase with test_make_aware_ambiguous_dst and test_get_table_ambiguous_dst in judge/models/tests/test_contest.py.
• Ran manage.py test across all related test suites with 18/18 passing tests.
• Ran flake8 with 0 warnings or errors.