Cron syntax in thirty seconds

A cron expression is five fields separated by spaces: minute (0–59), hour (0–23), day of month (1–31), month (1–12 or names), and day of week (0–7 or names, where both 0 and 7 are Sunday).

Each field takes * for every value, a number, a list like 1,15, a range like 9-17, or a step like */5 — and they combine, so 9-17/2 means every second hour from 9 through 17. Shortcuts like @daily and @hourly work too.

One classic surprise: when both day-of-month and day-of-week are restricted, standard cron runs the job when either matches, not both. This tool implements that rule, so the next-run times you see above are what a real cron daemon would do.