Pedro Müller

Passionate about clean code, modern web technologies and ai

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Python DateTime & Timezone

Rule of thumb: Store and compute in UTC. Convert to local only for display.

  • Naive vs aware: datetime.now() is naive (no tz). Prefer datetime.now(timezone.utc) or datetime.now(ZoneInfo("Europe/Lisbon")).
  • Python 3.9+: from zoneinfo import ZoneInfo (stdlib). Older: use pytz.
  • pytz: use tz.localize(dt) for local times; don’t pass tz into datetime(..., tzinfo=pytz.timezone(...)) for non-UTC (DST issues).
  • Store in DB: always in UTC. Validate dt.tzinfo is not None before save; convert with dt.astimezone(timezone.utc).
  • DST: ambiguous or non-existent times (e.g. 2:30 when clocks jump). pytz: AmbiguousTimeError / NonExistentTimeError. Handle explicitly (e.g. is_dst=False for ambiguous).
  • Testing: patch django.utils.timezone.now or equivalent to fix “now” and assert on conversions.

Debug: log or print dt, dt.tzinfo, and dt.astimezone(timezone.utc) so you see what’s actually there.