Cron and daylight saving: skipped and repeated times

A New York schedule crosses two different boundaries

For America/New_York in 2026, the spring transition is on 8 March. The local clock skips from 01:59:59 to 03:00:00. A request to run at 02:30 local time therefore needs a policy: skip it, shift it, or make it up later.

On 1 November the local 01:30 occurs at two different UTC instants. A scheduler might run once, run twice, or apply a misfire policy. The following table describes clock mappings, not a promise that every cron daemon executes at each entry.

Check the scheduler as well as the expression

Cronie documents special handling for small clock changes in its cron daemon: fixed-time jobs skipped during a forward jump can run after it, and repeated fixed-time jobs can be suppressed during a backward jump. Other implementations and managed services use their own rules. Do not infer operational behavior solely from a generic cron explanation.

PageUtil uses cron-parser for previews. Its timezone-aware interpretation belongs to that library and version. Quartz misfires, host restarts, queue delays and application locks are separate concerns. Verify the actual configured time zone and inspect logs containing an offset or a UTC timestamp.

Use Time Zone Converter to compare a local appointment with UTC. On an overlapping hour, inspect the ambiguity message instead of silently accepting the first interpretation. Browser timezone data also depends on the runtime’s installed database.

Make the requirement explicit

For “09:00 in London every working day”, use a named region zone and document the missing/repeated-time policy. A fixed UTC schedule changes its local display when the region changes offset.

For “every 24 elapsed hours”, choose an interval design and persist the last successful execution. This is different from “once per local calendar day”. Add idempotency keyed to the business operation so a repeated trigger cannot duplicate the work.

Before deployment, test both transitions, a restart after a missed run, and an execution lasting longer than the schedule interval. Keep the expression, scheduler version, zone and expected UTC results with the test evidence.

References

Use the linked specifications and product documentation to check behavior in your own environment.