Cron Explainer

A cron expression is five fields — minute, hour, day-of-month, month and day-of-week — that define a repeating schedule, used by Unix cron and many schedulers.

It translates the expression into plain English and lists the next few times it will run, so you can confirm a schedule does what you intended.

An asterisk (*) means "every", a slash (*/5) means "every 5", a dash (1-5) is a range, and a comma (1,15) is a list. This tool understands all of them.

By default cron uses the local time of the server it runs on, so the same expression can fire at a different clock time on machines in other zones. Always confirm the server timezone when a job must run at a specific local time.

Use a step value in the minute field, such as */15 in the minute position with an asterisk in the others, which fires at minutes 0, 15, 30 and 45 of every hour.

When both are set to specific values, standard cron runs the job when either matches, not only when both do. Leave one as an asterisk unless you understand this behavior, or the job may run more often than expected.

Classic cron does not run jobs it missed while the machine was off. Tools like anacron exist to catch up missed jobs on systems that are not always running, such as laptops.