Cron Explainer

What it's for

Understand a cron schedule before you rely on it. The tool describes it in plain English, lists the next ten times it will run, and converts it to the StartCalendarInterval that macOS launchd jobs use.

For example, when you’re setting up a scheduled job, or inherit a crontab line you don’t understand, check exactly when it fires.

How to use it

Type a cron expression with five fields: minute, hour, day of month, month and day of week. Lists (1,15), ranges (9-17), steps (*/15) and names such as jan and mon all work, as do the shortcuts @hourly, @daily, @weekly, @monthly and @yearly. The description and next runs update as you type, in your local time, and the launchd version is ready to copy into a plist.

To write one from scratch, open Build an expression: choose the minute, the hours and the days of the month from the menus (every, every N, at a list, or a range), and tick days of the week and months. The expression is written as you choose, with runs of days shortened to ranges such as 1-5 for Monday to Friday.

Example

Type this:

*/15 9-17 * * mon-fri

The tool describes it as “Every 15 minutes, during hours 9 through 17, on Monday through Friday.” The next runs start at 09:00 on the next weekday, then 09:15, 09:30 and so on.

Good to know

When both the day-of-month and day-of-week fields are set, the job runs if either matches, which surprises many people; cron and launchd both work this way. Six-field expressions with seconds aren’t supported.