Cron expression parser

The five fields

A cron expression is five values separated by spaces, read left to right: minute, hour, day of the month, month and day of the week. A * means every value. Linux crontab, GitHub Actions, Kubernetes CronJobs and most job schedulers use this format.

Commas list values (1,15), dashes give ranges (9-17), and a slash gives steps (*/10). The default example, */15 9-17 * * 1-5, runs every 15 minutes from 9:00 to 17:45 on weekdays. Some systems, such as Quartz and some cloud schedulers, add a sixth field for seconds at the front: */5 * * * * * runs every 5 seconds. The parser accepts both.

The traps

• Day of month and day of week together. 0 9 1 * 1 runs at 9:00 on the first of the month and on every Monday, not only on Mondays that fall on the first. The next runs list shows it at once.
• `9-17` includes 17. With */15 in the minute field, the last run is 17:45, not 17:00.
• Time zones. Servers often run cron in UTC. A job set for 9:00 runs at 9:00 UTC, which is 10:00 or 11:00 in Paris depending on the season. Pick the server's time zone in the list to see the real times.
• Daylight saving time. On the night clocks change, a job at 2:30 can run twice or not at all, depending on the scheduler. Avoid scheduling in the hour around 2:00 local time.

Checking an expression

Type or paste an expression and three things update: a sentence that describes it, a breakdown of each field, and a list of the next runs as dates. Seeing the actual dates is the surest check: if the list shows a run you did not expect, the expression does not mean what you thought. Change the number of runs to 50 to check a whole month of a frequent job, and switch the time zone to compare how it runs on a server elsewhere. A 24 hour clock and a more verbose wording are available.

Related tools

Cron logs and job systems usually record times as Unix timestamps; the timestamp converter turns them into dates in any time zone. Unix timestamps explained covers the format and its pitfalls.

If something goes wrong

• It says the expression is invalid. Count the fields: five, or six with seconds. A value is out of range, such as 24 for the hour or 60 for the minute.
• The next runs are not what you expected. Check the time zone, then the day of month and day of week fields.
• Your scheduler rejects an expression that works here. Schedulers differ: some do not accept names like MON, a seconds field, or special characters such as L and #.