Cron Syntax Compared: Unix, Quartz, AWS, GitHub Actions

POSIX defines them as minute (0-59), hour (0-23), day of the month (1-31), month (1-12) and day of the week (0-6, with 0 as Sunday). Each field is an asterisk for every value, a number, a range such as 1-5, or a comma-separated list.

That is all POSIX defines. Step values such as */5, month and day names, and 7 as a second way to write Sunday are extensions. cronie supports all three, plus ~ for a random value in a range and nicknames such as @daily and @reboot. An expression that uses only numbers, ranges, lists and asterisks is the most portable.

In Unix cron, the job runs when either one matches. POSIX states it, and the cronie manual gives the example in the last row of the table: 30 4 1,15 * 5 runs at 4:30 am on the 1st and 15th of each month, plus every Friday. It does not mean "the 1st and 15th, if a Friday".

Quartz and AWS avoid the question. Both add a ? character meaning "no specific value", and both require it in one of the two day fields.

A Quartz expression has six or seven fields. Seconds come first and an optional year comes last, so the five-field 0 9 * * 1-5 is not a valid Quartz expression. Days of the week are numbered 1 to 7 starting from Sunday, so 1-5 in Quartz means Sunday to Thursday. Use names such as MON-FRI to avoid the off-by-one.

Quartz also adds L for last, W for the nearest weekday and # for the nth weekday of the month.

An AWS expression is written cron(minutes hours day-of-month month day-of-week year): six required fields, no seconds, and a year from 1970 to 2199. You cannot put a value or * in both day fields; one of them must be ?. Days of the week are 1 to 7 or SUN to SAT, with 1 as Sunday. Expressions that would run more often than once a minute are not supported.

So the Unix 0 9 * * 1-5 becomes cron(0 9 ? * MON-FRI *). AWS's own examples for rules are in UTC, such as cron(0 10 * * ? *) for 10:00 am UTC every day. EventBridge Scheduler uses the same six fields and lets you set an IANA time zone for the schedule. AWS now labels scheduled rules as legacy and points to EventBridge Scheduler.

The schedule trigger takes POSIX cron syntax with five fields. GitHub documents four operators, *, ,, - and /, and accepts JAN-DEC and SUN-SAT as names. Nicknames such as @daily and @hourly are not supported.

Schedules run in UTC by default, and the shortest interval is once every 5 minutes. GitHub's documentation also describes an optional timezone key that takes an IANA time zone name, where that feature is available. It warns that scheduled runs can be delayed during periods of high load, including the start of every hour, so a job set for minute 0 may start late.

The one your scheduler documents. An expression copied from one system to another can be rejected or, worse, accepted with a different meaning, as with 1-5 in the day-of-week field.

When you can choose, start from a five-field expression that uses only numbers, ranges, lists and asterisks. It runs unchanged in Unix cron and GitHub Actions. For Quartz, put a seconds field in front, and for AWS add a year at the end; in both, replace one day field with ? and write weekdays as names.