PatternSoftwareLanguages & Runtimes
Python datetime: wall-clock subtraction and elapsed time diverge at a DST change
For elapsed time between real instants, convert aware timestamps to UTC before subtracting; preserve local-zone rules for calendar scheduling.
- Applies when
- Python applications distinguishing elapsed duration from local calendar scheduling
- Tested environment
- Python 3.14.7 and system IANA timezone data on macOS; Berlin transition assertions passed 2026-11-07
A timezone-aware value does not make every arithmetic operation an elapsed-time calculation. Python documents that subtraction ignores timezone adjustments when both operands have the same tzinfo object.
Local check: using one Europe/Berlin ZoneInfo object, 2026-03-29 01:30 to 03:30 produced a two-hour timedelta when subtracted directly. Converting both endpoints to UTC before subtraction produced one hour, matching the spring-forward transition. Both assertions passed.
The decision is semantic. “How long did this task run?” concerns elapsed instants. “Schedule the next meeting at the same local hour” concerns a local calendar rule. Treating these as interchangeable can shift appointments or misreport durations.
Store the relevant timezone identity for future local schedules, and decide how to handle ambiguous and nonexistent local times. A fixed UTC offset alone does not encode future daylight-saving rules.
Evidence: documented arithmetic plus one local transition example. This does not validate all zones, historical rules, recurrence systems, or future timezone legislation. Keep arithmetic expectations explicit in regression tests around transition boundaries.
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