Lee-W opened a new pull request, #75847:
URL: https://github.com/apache/airflow/pull/75847
## Why
An hourly cron that skips days, for example `0 * * * 1-5`, computes the next
run after Friday 23:00 by treating the wall-clock gap to Monday 00:00 as
elapsed time. That gap is 49 hours, but if a DST change falls over the weekend
the real elapsed time is 50 hours (clocks go back) or 48 hours (clocks go
forward). The next run is therefore an hour off:
| Timezone | current (UTC) | Before | After | After (local) |
|---|---|---|---|---|
| Europe/Berlin (fall back 2026-10-25) | 2026-10-23T21:00Z (Fri 23:00 CEST)
| 2026-10-25T22:00Z | 2026-10-25T23:00Z | Mon 00:00 CET |
| America/Chicago (fall back 2026-11-01) | 2026-10-31T04:00Z (Fri 23:00 CDT)
| 2026-11-02T05:00Z | 2026-11-02T06:00Z | Mon 00:00 CST |
| Australia/Sydney (fall back 2026-04-05) | 2026-04-03T12:00Z (Fri 23:00
AEDT) | 2026-04-05T13:00Z | 2026-04-05T14:00Z | Mon 00:00 AEST |
| Europe/Berlin (spring forward 2027-03-28) | 2027-03-26T22:00Z (Fri 23:00
CET) | 2027-03-28T23:00Z | 2027-03-28T22:00Z | Mon 00:00 CEST |
Before the fix the result is an hour early when clocks go back and an hour
late when clocks go forward.
The elapsed-time shortcut exists only so that an hourly schedule walks
through the repeated hour when clocks go back. It does not need to apply to
gaps longer than one hour.
## What
- Limit the elapsed-time shortcut in `CronMixin`
(`airflow-core/src/airflow/timetables/_cron.py`) to intervals of at most one
hour.
- For longer intervals, convert the wall-clock result directly to the
timezone. This keeps both occurrences of a repeated hour and ignores wall-clock
times that do not exist when clocks go forward.
- Add a helper, `_localize_all_folds`, that returns every valid localization
of a naive datetime. It relies on pendulum's `Timezone.convert()` honouring
`fold` (the project requires pendulum>=3.1.0); that was only verified against
the installed version, so the helper has its own unit test.
This does not fix every DST edge case. A brute-force comparison (below)
still found cases that are wrong both before and after this change: 234 for
`_get_next` and 106 for `_get_prev`, in zones such as Cairo, Lord_Howe,
Santiago and Beirut, with Lord_Howe and LHI (30-minute DST) making up most of
them. They are not introduced here and are not addressed in this PR; I suggest
tracking them in a separate issue.
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