Cron Every 30 Minutes

*/30 * * * *

Every hour at 00 and 30 minutes past

Field by Field

Field	Value	Matches
Minute	*/30	0, 30
Hour	*	every hour
Day of month	*	every day
Month	*	every month
Day of week	*	every day

Two Runs an Hour

*/30 fires at :00 and :30. The equivalent explicit form is 0,30 * * * *, which some teams prefer because it cannot be misread as "every thirty minutes from now".

A Good Default for Polling

Half-hourly is the sweet spot for a large class of jobs: syncing an external system, refreshing a cache, checking for new files in a bucket. It is frequent enough that staleness is rarely noticed and infrequent enough that the cost stays trivial.

Watch the DST Transitions

On a spring-forward day the 02:00–03:00 hour does not exist in local time, so a locally-scheduled job loses two runs. On a fall-back day that hour happens twice and the job runs four times. If either matters, schedule in UTC and convert at the edges.

Offsetting

7-59/30 * * * * fires at :07 and :37 instead, which keeps you off the busiest minutes of every shared API on the internet.

Time Zones

Standard crontab evaluates in system local time. Vercel Cron, GitHub Actions, Kubernetes CronJobs and AWS EventBridge all evaluate in UTC. If converting your local time crosses midnight, the day-of-week field shifts too — weekdays at 5pm in UTC-07:00 is 0 0 * * 2-6, not 0 0 * * 1-5.

The Day-Field Trap

When both the day-of-month and day-of-week fields are restricted, cron matches either, not both. 0 0 13 * 5 runs on every 13th *and* every Friday — not Friday the 13th. Leave one of them as * unless you genuinely want the union.

Other Common Schedules

Expression	Runs
* * * * *	Every minute
*/5 * * * *	Every 5 minutes
*/15 * * * *	Every 15 minutes
0 * * * *	Every hour, on the hour
0 0 * * *	Every day at midnight
0 9 * * *	Every day at 9am
0 9 * * 1-5	Weekdays at 9am
0 9 * * 1	Every Monday at 9am

Making the Job Safe

• Assume double firing. DST transitions, restarts and retries all cause it. Make the job

idempotent rather than assuming exactly-once.

• Assume overlap. Cron starts the next run whether or not the last one finished. Take a

lock.

• Avoid :00. Every hourly job on the internet fires at the top of the hour; a random

minute spreads load on shared APIs.

• Log the start and end. A cron job that silently stops running is invisible until

someone notices the missing output.