Programming Fundamentals › Programming Basics
Time Zone
Offsets from UTC that change with location and daylight saving.
Also known as: TZ, time zones, IANA time zone, tz database
A time zone is a region’s rule for what its clocks say relative to UTC. It’s more than a fixed number: the offset can change during the year (daylight saving time), and governments change the rules.
Offset vs zone name
| Example | Knows about daylight saving? | |
|---|---|---|
| Offset | +01:00 | No. It’s just the difference right now |
| Zone name (IANA) | Europe/Berlin | Yes, including past and future rule changes |
Prefer IANA names (America/New_York, Asia/Tokyo) when you need to remember where
someone is. An offset only describes one instant.
Avoid abbreviations like CST: the same letters are used for different zones in different
countries, so they’re ambiguous. Not every zone is a whole hour away from UTC either; India
is +05:30 and Nepal +05:45.
Converting
Keep the stored moment in UTC and convert when displaying:
from datetime import datetime, timezone
from zoneinfo import ZoneInfo
t = datetime(2024, 6, 1, 12, 0, tzinfo=timezone.utc)
t.astimezone(ZoneInfo("Asia/Tokyo")) # 2024-06-01 21:00:00+09:00
t.astimezone(ZoneInfo("Europe/Berlin")) # 2024-06-01 14:00:00+02:00Daylight saving oddities
When clocks go forward, some local times don’t exist (nothing happens between 02:00 and 03:00 that night in many places). When they go back, some local times happen twice. A job scheduled “at 02:30 local time” can be skipped or run twice, so schedule recurring server work in UTC.
Practical defaults: run servers in UTC, store timestamps in UTC, keep each user’s zone as a setting (or take it from their browser), and convert only to display. See dates and times for the full picture.