Cron Expression Builder / Explainer

What is a cron expression?

Cron is a time-based job scheduler used in Unix-like operating systems. A cron expression is a five-field string that specifies when a task should run. Fields represent, in order: minute, hour, day of month, month, and day of week.

Cron field quick reference

• Minute (0–59): the minute of the hour
• Hour (0–23): the hour of the day (24-hour clock)
• Day of month (1–31): specific day in the month
• Month (1–12 or JAN–DEC)
• Day of week (0–7 where 0 and 7 both mean Sunday, or SUN–SAT)

Special characters

• *: every possible value
• */n: every n-th value (e.g. */5 in the minute field = every 5 minutes)
• n-m: range from n to m (e.g. 1-5 in DOW = Monday through Friday)
• n,m: list (e.g. 1,3,5 in the month field = January, March, May)

Common cron examples

• * * * * *: every minute
• 0 * * * *: every hour, on the hour
• 0 0 * * *: every day at midnight
• 0 9 * * 1-5: Monday–Friday at 9:00am
• 0 0 1 * *: midnight on the 1st of every month
• */15 * * * *: every 15 minutes
• 0 0 1 1 *: midnight on January 1st (yearly)

DOM and DOW interaction

When both day-of-month and day-of-week are specified (neither is *), most cron implementations fire if either condition is satisfied. For example, 0 0 1 * 1 fires at midnight on the 1st of every month and also at midnight every Monday.

Quartz cron vs. Unix cron

Quartz (used in Java/Spring applications) extends the Unix cron format with additional fields. Unix cron uses 5 fields (minute, hour, day-of-month, month, day-of-week), while Quartz uses 6–7 fields by adding a seconds field at the start and an optional year field at the end. This causes frequent confusion when copying expressions between environments. A Unix expression like 0 9 * * 1 becomes 0 0 9 ? * MON in Quartz syntax.

Non-standard extensions

Many cron implementations support convenient shorthand strings instead of the 5-field format:

• @reboot: run once at startup
• @hourly: run at the beginning of every hour (0 * * * *)
• @daily / @midnight: run at midnight each day
• @weekly: run at midnight every Sunday
• @monthly: run at midnight on the first of each month
• @yearly / @annually: run at midnight on January 1st

These are supported by Vixie cron (Linux), cronie, and many cloud schedulers, but are not part of the POSIX standard.

Common mistakes

• Month is 1-indexed: January = 1, December = 12. A common error is writing 0 0 * 0 * to mean "every month" when 0 is invalid - use *.
• Day-of-week is 0–7: In Unix cron, both 0 and 7 represent Sunday. In Quartz cron, 1 = Sunday and 7 = Saturday (1-indexed). This discrepancy is a common source of bugs when migrating expressions.
• Assuming simultaneous firing: */15 8-18 * * 1-5 does not fire exactly at 8:00, 8:15, etc. - it fires at each 15-minute mark within the hour (0, 15, 30, 45) during hours 8 through 18 on weekdays.