Reading and writing cron schedules
The five cron fields, common expression patterns, and the timezone and overlap issues that make scheduled jobs misfire. Local tools available on wowTools.
A standard cron expression has five fields: minute, hour, day of month, month, and day of week. Each accepts a value, a list, a range, a step, or an asterisk meaning every value. That small grammar produces schedules that are compact to write and surprisingly easy to misread.
Patterns worth recognising
*/15 * * * * runs every fifteen minutes. 0 3 * * * runs at three in the morning. 0 9 * * 1-5 runs on weekday mornings. The awkward case is day-of-month combined with day-of-week, where many implementations treat the two as an OR rather than an AND.
Timezones decide when it really runs
Cron interprets wall-clock time, so the schedule depends on the timezone of the machine or scheduler configuration. Running in UTC removes daylight-saving ambiguity but shifts the local hour twice a year; pinning a local zone keeps the hour and inherits the transitions.
Overlap and idempotency
Nothing in cron prevents a run from starting while the previous one is still going. Long jobs need a lock, and every job should be safe to run twice, because a backward daylight-saving transition repeats an hour.
Read the expression before you deploy
Translate any unfamiliar schedule into plain language with Cron Explainer, check instants with Timestamp Converter, and weigh clock-based against elapsed-time scheduling in Cron vs Interval Scheduling.
Try related tools
Keep exploring
- learnTimestamps, epochs, and timezonesUnix time, seconds versus milliseconds, UTC offsets, and the daylight-saving edge cases that break scheduled work. Local tools available on wowTools.
- learnWorking with Cron ExplainerTurn cron expressions into readable schedules. A practical guide to Cron Explainer: when it fits, how to get a reliable result, and the mistakes worth avoiding.
- compareCron vs Interval SchedulingWall-clock cron expressions versus fixed intervals—compare drift, timezone handling, and predictability for scheduled jobs.
- glossaryCron expressionA cron expression schedules recurring work using five fields—minute, hour, day of month, month, and day of week—interpreted against wall-clock time. Related wowTools tools and further reading are linked from this glossary entry.