Coldwast
Cron expression explainer
Paste a crontab line. You get it in plain English and the next times it will actually fire, computed in your own timezone. Everything runs in your browser.
The rule that surprises people
When day of month and day of week are both restricted, they combine with OR, not AND. 0 0 1 * 1 does not mean "the 1st, if it is a Monday". It means every 1st of the month, and every Monday as well. The specification is explicit: "if either the month or day of month is specified as an element or list, and the day of week is also specified as an element or list, then any day matching either the month and day of month, or the day of week, shall be matched."
What it does not know
The next runs are computed in your browser's timezone. A server usually runs in UTC or its own local zone, so the wall-clock times there may differ. Daylight saving transitions are another edge: a job scheduled inside the hour skipped in spring does not run that day, and one inside the repeated autumn hour can run twice, depending on the implementation.