Cron every 3 days

It runs at 09:00 on the 1st, 4th, 7th, through 28th of each month. It is a calendar pattern, not a continuous 72-hour interval. Expression: 0 9 1-30/3 * *

1-30/3 selects 1, 4, …, 28 and excludes the 31st. */3 also selects the 31st when present; both reset on the next 1st, so neither is a strict 72-hour interval. Expression: 0 0 1-30/3 * *

Trigger daily in UTC, then gate with day=$(( $(date +%s) / 86400 )); (( day % 3 == 0 )) || exit 0. Unix-day modulo continues across month and year boundaries, keeping selected UTC midnights 72 hours apart.

Unix-day numbering is continuous, while date +%j resets to 001 each January. Because 365 is not divisible by three, day-of-year modulo changes the phase at New Year.

Yes. The same local time three dates later can be 71 or 73 elapsed hours away across a DST change. Use UTC epoch-day modulo for exact 72-hour spacing.

Use cron(0 0 * * ? *) as a daily UTC trigger, then apply Unix-day modulo three in the target. EventBridge day-of-month steps reset monthly. Expression: 0 0 * * ? *

Use 0 0 0 * * ? to trigger daily, then gate the job with epoch-day modulo three or persisted state. Quartz calendar fields cannot maintain a strict three-day interval across months. Expression: 0 0 0 * * ?

Yes. Schedule the workflow daily and skip days whose Unix-day number is not divisible by three. GitHub Actions defaults to UTC, and its five-minute floor does not affect daily scheduling.

Yes. The daily UTC trigger fits Vercel Hobby's once-per-day limit; the endpoint exits unless the Unix-day modulo check selects that date.

Write the last-success timestamp only after completion and protect the check with a lock. A pre-run update can turn one failed attempt into a six-day wait.

Yes. Crontap can run the daily HTTP trigger with retries, while the endpoint's modulo check or persisted state enforces the three-day interval.

0 0 1-30/3 * * matches calendar dates 1, 4, …, 28 and resets on the 1st; it is not a strict 72-hour interval. For strict spacing, trigger daily in UTC and gate with epoch-day modulo 3.

The editor shows the next 30+ matches on a calendar. Check dates around the 1st: this day-of-month step resets monthly, while a strict 72-hour cadence requires a daily UTC trigger and epoch-day modulo 3.

Linux cron, Vercel, node-cron, robfig, and Kubernetes accept this five-field calendar pattern, but it resets monthly everywhere. A strict 72-hour cadence needs a daily UTC trigger plus epoch-day modulo 3; AWS EventBridge and Quartz require six-field conversions.

Most schedulers will start the new run anyway, leading to overlap. To prevent this, wrap the command with flock -n /tmp/myjob.lock /path/to/script.sh on Linux, or set concurrencyPolicy: Forbid on Kubernetes CronJob. Vercel and EventBridge may overlap runs if the previous one is still executing; make the handler idempotent.