Every hour cron: what 0 * * * * means and why the top of the hour is crowded

Reading 0 * * * *

The minute field is a literal 0 and every other field is a wildcard, so the job runs at minute 0 of every hour: 24 times a day. The macro @hourly means the same thing in Vixie cron, cronie and Kubernetes.

The parser on this page reads it as: Every hour at :00. 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.

The classic mistake is * */1 * * *. That puts the wildcard in the minute field, so it matches every minute of every hour, sixty runs an hour instead of one. The hour step is redundant; the minute field is the one that has to be pinned.

A crontab line

0 * * * * /usr/bin/logrotate /etc/logrotate.d/app-hourly is a typical hourly job. If the work can take longer than an hour, guard it with flock -n /run/lock/app-hourly.lock in front of the command so the next run exits instead of overlapping.

The top of the hour is crowded

Minute 0 is the most popular minute in every crontab. Backups, report jobs, certificate checks and cache refreshes all land there, and on shared infrastructure that turns into a load spike every hour. GitHub explicitly warns that scheduled workflows are most likely to be delayed or dropped at the start of an hour. Picking an odd minute, such as 17 * * * *, keeps the hourly cadence and dodges the queue.

Daylight saving affects hourly jobs mildly: the skipped hour loses one run and the repeated hour may run twice, depending on the cron implementation. If each run processes the previous hour of data, compute the window from UTC timestamps rather than from the local clock.

systemd, Kubernetes and GitHub Actions

The systemd timer is OnCalendar=hourly, the same as *-*-* *:00:00; add Persistent=true if a run missed while the machine was off should happen at the next boot. Kubernetes accepts both 0 * * * * and @hourly. GitHub Actions accepts 0 * * * * in UTC but does not support the @hourly macro. Neighbouring schedules: every 30 minutes doubles the runs, every day at midnight reduces them to one.