A cron expression selects calendar times; it does not declare a timezone or wait for an earlier job to finish. This guide uses numeric five-field cron and the toolbox's UTC preview. Check the destination scheduler's field count and timezone support before enabling a job.
1. Write the requirement in plain language
“Weekdays at 09:00 UTC” is a different requirement from “weekdays at 09:00 UK local time throughout the year”. UK local time follows GMT in winter and British Summer Time, one hour ahead of UTC, in summer. Avoid an unqualified “9am” in a change request.
2. Build a weekday UTC schedule
0 9 * * 1-5
The fields mean minute 0, hour 9, any day of month, any month and Monday through Friday. Open the Cron Builder worked examples and load “Weekdays at 09:00 UTC”. Review the five fields and select Build. The expression is not a complete command or service configuration.
In the Schedule Visualiser, use that expression and start time 2026-11-02T08:59:00Z. The first three matches are 2026-11-02T09:00:00.000Z, 2026-11-03T09:00:00.000Z and 2026-11-04T09:00:00.000Z. Matches are strictly after the supplied start: starting exactly at 09:00 excludes that instant. The preview returns up to 12 matches within approximately one year.
3. Separate UTC from UK local time
Scroll the table sideways if any columns are hidden.
| UTC run | UK local time |
|---|---|
| 15 January 2026, 09:00 UTC | 09:00 GMT |
| 15 July 2026, 09:00 UTC | 10:00 BST |
For a year-round 09:00 UK-local requirement, use a scheduler that supports Europe/London and review its daylight-saving behaviour. Do not treat this UTC-only preview as a simulation of that scheduler. For implementations following the linked crontab manual, a missing local clock time may not run and a repeated time may run twice; other schedulers can handle transitions differently.
4. Check day matching and overlap
0 9 1 * 1 does not mean “the first Monday” in this tool. With both day fields restricted, it matches the first of the month or a Monday, when the other fields match. Compare the explanation in Cron Expression Explainer with the actual next-run list.
The quarter-hour example */15 * * * * matches minutes 0, 15, 30 and 45. Starting at 2026-11-02T09:07:00Z gives first matches at 09:15, 09:30 and 09:45 UTC. If a job takes 20 minutes, new runs can overlap. Configure appropriate locking or concurrency control, a timeout, failure alerts and a recovery procedure in the real system.
Keep practising
The Cron lesson checks your understanding. These tools do not execute jobs, model downtime, prevent duplicate processing or validate your command's permissions.