Cron Every 20 Minutes

*/20 * * * *

Every hour at 00, 20 and 40 minutes past

Field by Field

Field	Value	Matches
Minute	*/20	0, 20, 40
Hour	*	every hour
Day of month	*	every day
Month	*	every month
Day of week	*	every day

Three Runs an Hour

*/20 matches minutes 0, 20 and 40. Twenty divides 60 exactly, so the spacing is genuinely even — unlike */7, which fires at 0, 7, 14, 21, 28, 35, 42, 49 and 56 and then waits four minutes for the next hour.

Restarts Do Not Shift the Schedule

Cron matches wall-clock time rather than counting from process start, so restarting the scheduler at 10:13 does not move the next run to 10:33 — it stays at 10:20. That is usually what you want, and it is the opposite of how an interval timer behaves.

Choosing Between 15 and 20

Fifteen minutes gives four runs an hour and aligns with quarter-hour boundaries that humans reason about. Twenty gives three and is 25% cheaper in invocations. If the work is metered, that difference is real; if it is not, pick the one that matches how stale the output is allowed to be.

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.

Deploying This Schedule

``bash # crontab -e (system local time) */20 * * * * /usr/local/bin/job.sh >> /var/log/job.log 2>&1 `

`json // vercel.json (UTC) { "crons": [{ "path": "/api/job", "schedule": "*/20 * * * *" }] } `

`yaml # .github/workflows/job.yml (UTC) on: schedule: - cron: '*/20 * * * *' `

Starting from midnight UTC on 1 January 2026, this fires at:

Next runs
2026-01-01 00:20 UTC
2026-01-01 00:40 UTC
2026-01-01 01:00 UTC
2026-01-01 01:20 UTC

Every hour at 00, 20 and 40 minutes past.

Making It Survive Production

• Redirect output. A cron job with no redirect emails its output to the crontab owner,

and on most servers that mail goes nowhere. Log to a file instead.

• Use absolute paths. Cron runs with a minimal environment — no PATH you recognise,

no shell profile, no nvm. A script that works in your terminal frequently fails here for exactly that reason.

• Escape percent signs. In a crontab, % means newline. date +\%Y needs the

backslash or the command truncates at the percent.

• Take a lock. Cron starts the next run whether or not the last finished. flock` is

one line and prevents the overlap.

• Alert on absence. A job that stops running is silent. Ping a dead-man's-switch service

on success so the *lack* of a ping is what pages you.