Time Zones with zoneinfo

18 min · Intermediate

A meeting at 9:00 means different moments in Mumbai and New York. A datetime without a zone is only a clock reading. With a zone, it names one exact moment. Python's zoneinfo module supplies the zones.

Naive and aware datetimes

Aware datetime is a datetime that has a time zone attached, so it names one exact moment. A naive datetime has no zone and is only a reading on a clock.

A naive datetime has no zone, so it is only a reading on a clock. An aware datetime carries a zone, so it names one exact moment. Python refuses to compare a naive with an aware one.

Python

from datetime import datetime, timezone
naive = datetime(2026, 11, 8, 9, 0)aware = datetime(2026, 11, 8, 9, 0, tzinfo=timezone.utc)print(naive.tzinfo, aware.tzinfo)

Example 1 · Naive versus aware

Python

1from datetime import datetime, timezone2
3naive = datetime(2026, 11, 8, 9, 0)4aware = datetime(2026, 11, 8, 9, 0, tzinfo=timezone.utc)5print(naive.tzinfo, aware.tzinfo)6try:7    print(naive < aware)8except TypeError as err:9    print("TypeError:", err)

Output

None UTC
TypeError: can't compare offset-naive and offset-aware datetimes

What this code does

The naive value has no zone, the aware one has UTC.
Python refuses to compare them rather than guess.

Comparing naive < aware raises TypeError. That is a feature: it stops a silent mix-up.

ZoneInfo and astimezone

ZoneInfo("Asia/Kolkata") gives a zone by its standard name. astimezone(zone) shows the same moment on another zone's clock. The moment does not change; only the clock reading does.

Python

from datetime import datetimefrom zoneinfo import ZoneInfo
utc = datetime(2026, 11, 8, 9, 0, tzinfo=ZoneInfo("UTC"))print(utc.astimezone(ZoneInfo("Asia/Kolkata")))

One moment, 09:00 UTC on 8 November 2026, shown on three clocks: Kolkata 14:30 with offset +05:30, London 09:00 with offset +00:00 and New York 04:00 with offset -05:00.

Example 2 · One moment, three clocks

Python

1from datetime import datetime2from zoneinfo import ZoneInfo3
4utc = datetime(2026, 11, 8, 9, 0, tzinfo=ZoneInfo("UTC"))5for name in ["Asia/Kolkata", "Europe/London", "America/New_York"]:6    local = utc.astimezone(ZoneInfo(name))7    print(name, local.strftime("%H:%M %z"))8print(utc == utc.astimezone(ZoneInfo("Asia/Kolkata")))

Output

Asia/Kolkata 14:30 +0530
Europe/London 09:00 +0000
America/New_York 04:00 -0500
True

What this code does

%z shows the offset from UTC: +0530 for Kolkata, -0500 for New York in November.
The two datetimes are equal because they are the same moment.

On Windows, run pip install tzdata once so Python has the zone database.

Store UTC, show local

UTC is the world reference time with no daylight saving and a fixed offset of zero. Systems store timestamps in UTC and convert to a local zone for display.

Save timestamps in UTC, which never shifts for daylight saving. Convert to the viewer's zone only when you display them. Use replace(tzinfo=...) only to label a naive value whose zone you already know.

Python

from datetime import datetimefrom zoneinfo import ZoneInfo
local = datetime(2026, 11, 8, 18, 0, tzinfo=ZoneInfo("Europe/London"))print(local.astimezone(ZoneInfo("UTC")).isoformat())

Example 3 · Label, then convert

Python

1from datetime import datetime2from zoneinfo import ZoneInfo3
4naive = datetime(2026, 11, 8, 9, 0)5labelled = naive.replace(tzinfo=ZoneInfo("Asia/Kolkata"))6print(labelled.hour)7print(labelled.astimezone(ZoneInfo("UTC")).isoformat())

Output

9
2026-11-08T03:30:00+00:00

What this code does

replace kept 9 on the clock and only added the zone.
Converting to UTC shows the real moment, 03:30 UTC.

replace relabels the clock reading. astimezone converts the moment. They are not the same.

Daylight saving traps

In some zones clocks jump an hour twice a year. Adding a timedelta to an aware datetime moves the clock reading, not the real elapsed time. For exact durations, convert to UTC first, add, then convert back.

Python

1from datetime import datetime, timedelta2from zoneinfo import ZoneInfo3
4ny = ZoneInfo("America/New_York")5start = datetime(2026, 3, 7, 12, 0, tzinfo=ny)6print(start + timedelta(days=1))

Example 4 · Add a day across daylight saving

Python

1from datetime import datetime, timedelta2from zoneinfo import ZoneInfo3
4ny = ZoneInfo("America/New_York")5start = datetime(2026, 3, 7, 12, 0, tzinfo=ny)6print(start + timedelta(days=1))7real = start.astimezone(ZoneInfo("UTC")) + timedelta(hours=24)8print(real.astimezone(ny))

Output

2026-03-08 12:00:00-04:00
2026-03-08 13:00:00-04:00

What this code does

The clocks in New York moved forward on 8 March 2026.
Adding 1 day keeps the clock at 12:00. Adding 24 real hours through UTC gives 13:00 local time.

This keeps 12:00 on the clock, even though only 23 real hours passed.

Next step

Check your understanding

11 quick questions

Naive and aware datetimes

Aware datetime is a datetime that has a time zone attached, so it names one exact moment. A naive datetime has no zone and is only a reading on a clock.

A naive datetime has no zone, so it is only a reading on a clock. An aware datetime carries a zone, so it names one exact moment. Python refuses to compare a naive with an aware one.

1from datetime import datetime, timezone2 3naive = datetime(2026, 11, 8, 9, 0)4aware = datetime(2026, 11, 8, 9, 0, tzinfo=timezone.utc)5print(naive.tzinfo, aware.tzinfo)6try:7 print(naive < aware)8except TypeError as err:9 print("TypeError:", err)

1from datetime import datetime2from zoneinfo import ZoneInfo3 4utc = datetime(2026, 11, 8, 9, 0, tzinfo=ZoneInfo("UTC"))5for name in ["Asia/Kolkata", "Europe/London", "America/New_York"]:6 local = utc.astimezone(ZoneInfo(name))7 print(name, local.strftime("%H:%M %z"))8print(utc == utc.astimezone(ZoneInfo("Asia/Kolkata")))

Store UTC, show local

UTC is the world reference time with no daylight saving and a fixed offset of zero. Systems store timestamps in UTC and convert to a local zone for display.

1from datetime import datetime2from zoneinfo import ZoneInfo3 4naive = datetime(2026, 11, 8, 9, 0)5labelled = naive.replace(tzinfo=ZoneInfo("Asia/Kolkata"))6print(labelled.hour)7print(labelled.astimezone(ZoneInfo("UTC")).isoformat())

1from datetime import datetime, timedelta2from zoneinfo import ZoneInfo3 4ny = ZoneInfo("America/New_York")5start = datetime(2026, 3, 7, 12, 0, tzinfo=ny)6print(start + timedelta(days=1))7real = start.astimezone(ZoneInfo("UTC")) + timedelta(hours=24)8print(real.astimezone(ny))