Cron every 3 hours at :15: the expression, explained, in Next.js / Vercel

The cron expression for “every 3 hours at :15” is 15 */3 * * *. This page shows what each field does, the next five times it will fire, the mistake most people make with it, and a working Next.js / Vercel example you can paste.

The expression

15 */3 * * *

Field	Value	Meaning
minute	15	exactly 15
hour	*/3	every 3 hours, starting at 0
day of month	*	every day of month
month	*	every month
day of week	*	every day of week

Next five runs (UTC, from now)

2026-09-18 00:15 UTC
2026-09-18 03:15 UTC
2026-09-18 06:15 UTC
2026-09-18 09:15 UTC
2026-09-18 12:15 UTC

Mistakes people make with this schedule

Using "* */3 * * *" for every 3 hours. That runs every minute of every 3th hour. The minute field must be the fixed number 15.
Assuming the server clock is your clock. Cron runs in the machine’s timezone unless the runtime says otherwise; the examples here are UTC.
No lock around a job that might overlap itself, and no alert when it silently stops. That second one is what Supercrontab is for.

Run it in your stack

Linux crontab Laravel WordPress Next.js / Vercel Node.js Python Docker GitHub Actions

// vercel.json
{
  "crons": [
    { "path": "/api/cron/run", "schedule": "15 */3 * * *" }
  ]
}

Vercel crons run in UTC and, on the Hobby plan, at most once a day. For anything more frequent call the route from Supercrontab.

Questions

Does 15 */3 * * * run in my local time?

Only if the scheduler is configured for it. crontab uses the system timezone; Vercel and GitHub Actions always use UTC; Laravel and node-cron take a timezone option. In Supercrontab the timezone is a field on the job.

What if the previous run is still going?

Classic cron starts the next run anyway. Add a lock (flock on Linux, withoutOverlapping() in Laravel) or let Supercrontab skip the tick while the previous request is open.

Is there a shorter way to write this?

No. The five-field form is the shortest portable version.