Gaps and overlaps
When clocks spring forward, a range of wall-clock times does not happen — in New York 02:00–02:59 is skipped. When they fall back, an hour happens twice. A job scheduled inside either window behaves differently per platform. cronie and Vixie cron run a skipped fixed-time job right after the change and do not repeat it on fall-back. Kubernetes (robfig/cron) skips it and can run twice. AWS EventBridge Scheduler documents skip-and-run-once. Quartz documents that firings may be missed or duplicated.
The DST tab lists every clock change in the next two years for your zone, the runs it affects, and a table comparing how each policy would behave. Switch the policy in the side panel to match your platform; the timetable and exports follow it. Confidence labels say whether a policy comes from platform documentation, from reading the scheduler’s source, or is unverified.
Avoiding the problem
Schedule critical jobs outside 01:00–03:00 local time, run them in UTC, or make them idempotent so a duplicate run is harmless. Southern-hemisphere zones change in the opposite months, and Lord Howe Island shifts by 30 minutes.
Try these
Questions
Does a UTC schedule have DST problems?
Not in when it runs — UTC has no DST. But the local wall-clock time it corresponds to moves by an hour twice a year, which matters for business-hours jobs.
Why does cronie treat */30 differently from 30 2?
cronie classifies jobs whose minute or hour field starts with * as wildcard jobs. They simply follow the wall clock; fixed-time jobs get catch-up and no-repeat handling.