Cron Every 12 Hours

0 */12 * * *

At 00:00 and 12:00

Field by Field

Field	Value	Matches
Minute	0	0
Hour	*/12	0, 12
Day of month	*	every day
Month	*	every month
Day of week	*	every day

Midnight and Noon

0 */12 * * * fires at 00:00 and 12:00. It is equivalent to 0 0,12 * * *, which states the intent more plainly and is easier to change later.

Choose the Hours Deliberately

Midnight and noon are the default rather than the best choice. Midnight is the busiest hour on almost every shared system — every daily job in every time zone converges on it — and noon collides with peak traffic for consumer products.

0 3,15 * * * gives the same twice-daily rhythm in two much quieter hours.

Twelve Hours Is a Long Time to Be Wrong

At this spacing a silent failure can go unnoticed for most of a day. Emit a heartbeat on success and alert on its absence, rather than alerting on failure — a job that never starts produces no failure to alert on.

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) 0 */12 * * * /usr/local/bin/job.sh >> /var/log/job.log 2>&1 `

`json // vercel.json (UTC) { "crons": [{ "path": "/api/job", "schedule": "0 */12 * * *" }] } `

`yaml # .github/workflows/job.yml (UTC) on: schedule: - cron: '0 */12 * * *' `

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

Next runs
2026-01-01 12:00 UTC
2026-01-02 00:00 UTC
2026-01-02 12:00 UTC
2026-01-03 00:00 UTC

At 00:00 and 12:00.

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.