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Adding the check-in margin to the next expected check-in in the monitor's timezone uses wall-clock arithmetic, which resets the datetime's fold. During the repeated hour after DST ends, next_checkin_latest then lands an hour early, so check_missed marks every slot missed although the check-in arrives on time. 🤖 Generated with Claude Code
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October 25, 2026 09:51
Python returns False for an inter-zone == when one operand is an ambiguous local time with fold=1 (PEP 495). 🤖 Generated with Claude Code
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Crontab monitors in a DST timezone report every check-in as missed during the repeated hour after DST ends.
get_next_expected_checkin_latestnow adds the check-in margin in UTC instead of local time.Local datetime arithmetic resets the fold, so the margin deadline lands an hour early
get_next_expected_checkinreturns the next slot in the monitor's timezone.In the repeated hour that datetime carries
fold=1.Adding a
timedeltato it is wall-clock arithmetic and returnsfold=0, the first pass of that hour.With
* * * * *,Europe/Berlinand a 5 minute margin, a check-in at 2024-10-27 01:00:03 UTC gives:next_checkinnext_checkin_latestbeforenext_checkin_latestaftercheck_missedseesnext_checkin_latestalready in the past and creates a missed check-in for the slot at once.The real check-in arrives seconds later as
ok, but withfailure_issue_threshold: 1every slot from 01:00 to 01:54 UTC opened a monitor failure issue.Slots whose deadline falls after the repeated hour, from 02:55 local time on, are unaffected.
Observed on a self-hosted instance on 2026-10-25
Every minutely and five-minutely monitor in
Europe/Berlinshowed amissedcheck-in exactly at each expected time from 01:00 to 01:54 UTC, next to anokcheck-in for the same slot about 3 seconds later.No phantom misses appeared before or after that window.
Spring forward is unaffected: a skipped local time resolves to the correct UTC instant.
Related: #66868 fixed DST handling in the schedule itself
Replacing croniter with cronsim made
get_next_scheduleandget_prev_schedulecorrect across DST.This bug sits after the schedule, in the margin addition.
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