0 6 1-7,15-21 * 1
At 06:00 on Monday on day 1, 2, 3, 4, 5, 6, 7, 15, 16, 17, 18, 19, 20 and 21 of the month — note that cron matches either day field, so this fires on whichever comes first, not only when both agree
Field by Field
| Field | Value | Matches |
|---|---|---|
| Minute | 0 | 0 |
| Hour | 6 | 6 |
| Day of month | 1-7,15-21 | 1, 2, 3, 4, 5, 6, 7, 15, 16, 17, 18, 19, 20, 21 |
| Month | * | every month |
| Day of week | 1 | 1 |
Cron Has No Fortnight
Cron's fields are minute, hour, day-of-month, month and day-of-week. None of them counts weeks, so "every two weeks" cannot be written directly.
The expression above approximates it: Mondays that fall in the 1st–7th or 15th–21st of the month — the 1st and 3rd Monday. That is close to fortnightly but drifts, because months contain four or five weeks.
The Exact Version Needs a Guard
For a true 14-day cycle, run weekly and check the ISO week number:
``bash
0 6 * * 1 [ $(( $(date +%V) % 2 )) -eq 0 ] && /opt/app/biweekly.sh
`
Even weeks only, which is exactly every fourteen days — with one caveat: ISO week 53 exists
in some years, so the parity flips at that boundary. For payroll, anchor to a known start
date instead:
`bash
0 6 * * 1 [ $(( ( $(date +%s) / 86400 - 20000 ) / 7 % 2 )) -eq 0 ] && ./run.sh
`
Semi-Monthly Is Not Biweekly
Twice a month is 24 runs a year; every two weeks is 26. Payroll systems that conflate them
produce two unexpected pay periods, usually noticed in December.
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
- Log the start and end. A cron job that silently stops running is invisible until