Cron expression parser and explainer

*/5 means every 5th value of the field, counted from the field's first value. In the minute field it fires at minutes 0, 5, 10 … 55, which is 12 times an hour. A step can follow a range too: 10-30/10 gives minutes 10, 20 and 30. Steps restart each hour, so */7 runs at minute 56 and then again at minute 0.

Write 0 0 * * *, which means minute 0 of hour 0 on every day, or use the @daily macro, which cronie and Vixie cron expand to the same five fields. For 2:30 a.m. every day write 30 2 * * *, and for 09:00 on weekdays write 0 9 * * 1-5. The hour is read in the server's time zone unless the crontab sets CRON_TZ.

If both fields are restricted, cron runs when either one matches, so 0 12 1 * MON fires at noon on the 1st of every month and on every Monday. In Vixie cron and cronie, a day field that starts with * switches this to both, which is why 0 0 */2 * FRI runs only on Fridays that fall on odd dates such as the 23rd.

Standard cron uses 5 fields. Quartz and Spring schedulers put a seconds field in front, making 6, and Quartz accepts an optional year as a 7th, so an expression like 0 0/15 * * * ? belongs to those tools rather than crontab; the ? placeholder is Quartz syntax. This parser reads the 5-field crontab format plus the @hourly, @daily, @weekly, @monthly and @yearly macros.

Not directly: the smallest unit in a crontab is 1 minute, because the first field is the minute. The usual workaround is two entries, * * * * * job and * * * * * sleep 30; job, which start the job at 0 and 30 seconds past each minute. systemd timers accept OnCalendar schedules down to the second if you need finer control.

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 7 worked examples whose answers come from independent sources; for example, “Every 15 minutes in office hours” is checked against Brute-force Python 3.8 datetime scan minute by minute with an independent field parser; 4 minutes × 9 hours = 36 runs a day; 09:00 − 00:00 = 32400 s.

POSIX.1-2017 (IEEE Std 1003.1) — crontab utility, input file format; crontab(5) — cronie / Vixie cron: ranges, steps, names, @ macros and the day-of-month / day-of-week rule.