Cron Expression Explainer

A cron expression is a 5-field string that defines a schedule for recurring tasks. The fields, left to right, represent: Minute (0-59), Hour (0-23), Day of Month (1-31), Month (1-12), and Day of Week (0-7, where both 0 and 7 represent Sunday). Use * for 'every', */n for 'every n units', and ranges like 1-5 or lists like 1,3,5.

An asterisk means 'any value' or 'every possible value' for that field. So '* * * * *' runs every minute of every hour of every day. '0 * * * *' runs at minute 0 of every hour (i.e. once per hour). '0 9 * * *' runs at 9:00 AM every day.

Both produce the same result: they run at minutes 0, 15, 30, and 45 of every hour. The step syntax (*/15) is more concise and readable for evenly-spaced intervals. The list syntax (0,15,30,45) is more explicit and useful for irregular intervals like 0,20,50.

By default, cron on Linux runs in the server's local timezone (set by /etc/timezone or TZ env var). GitHub Actions cron runs in UTC. AWS EventBridge uses UTC by default but supports timezone scheduling. Always check your scheduler's timezone setting, especially for daylight saving time transitions.

Standard cron cannot schedule sub-minute intervals — the minimum is 1 minute (* * * * *). For more frequent execution, use a sleep loop inside your script (e.g., while true; do task; sleep 10; done), systemd timers with sub-second precision, or dedicated task schedulers that support finer granularity.