Every 15 minutes cron: what */15 * * * * means, with a quarter-hour crontab example

Field by field

The minute field */15 steps through 0–59 in fifteens, which yields 0, 15, 30 and 45. Hour, day of month, month and day of week are all asterisks, so the four quarter-hour slots repeat around the clock: 96 runs per day. Writing the list out as 0,15,30,45 * * * * is identical and some teams prefer it because nobody has to remember how steps are anchored.

The parser on this page reads the expression as: Every 15 minutes. The next run times in the panel above are computed in your browser, in your own timezone, so they are not printed in this text.

A quarter-hour job in a crontab

A common use is collecting metrics or rotating short-lived caches: */15 * * * * /usr/local/bin/export-usage --since 15m >> /var/log/export-usage.log 2>&1. Notice the job asks for the last 15 minutes of data. If one run fails or the host is down, that window is lost; a job that records the last timestamp it processed recovers by itself.

Remember that % is special in a crontab line (it becomes a newline), so a date +%F inside the command must be written date +\%F.

Timezones and overlap

Interval schedules such as this one are the least affected by daylight saving time: when the clocks jump forward an hour, the four runs of the skipped hour simply never happen, and on the fall-back day the repeated hour usually produces four extra runs. A job that sums usage per run will double-count that hour unless it works from timestamps.

If a run can take longer than 15 minutes, guard it with flock -n or, in a Kubernetes CronJob, concurrencyPolicy: Forbid. Replace is the other option there: it kills the running Job and starts the new one, which suits jobs where only the freshest result matters.

systemd and GitHub Actions

The systemd timer form is OnCalendar=*:0/15, optionally with RandomizedDelaySec=60 to stop a fleet hitting a shared service in the same second. In GitHub Actions, cron: '*/15 * * * *' is valid and is evaluated in UTC; for a related but lighter schedule, every 30 minutes halves the runs, and every 5 minutes triples them.