Cron Expression Generator

What does this cron tool do?

It translates a cron line into plain English as you type, and then does what a cron editor usually leaves to you: it reads the line back for the traps that fire without any error message.

• The OR trap, detected. When both day fields are restricted, cron runs when either matches. The page says so, in words, before you deploy it.
• Three platforms, not one. Unix and Kubernetes (5 fields), Quartz (6 or 7, seconds first), AWS EventBridge (6, year last, question mark required).
• The day-of-week shift. Write a digit and the page tells you what that same digit means on the other platforms, because it is not the same day.
• Next runs in any IANA timezone, with that zone's real daylight saving rules.
• Daylight saving warnings on the actual dates when a run is skipped or happens twice.
• Two-way editing. Change the whole line, or one field at a time.
• Nothing leaves your browser. A schedule quietly describes how an internal system works.

The OR trap, which is the famous one

0 0 13 * 5 looks like "midnight on Friday the 13th". It is not.

When day-of-month and day-of-week are both restricted, they combine with OR. POSIX specifies it and Vixie cron implements it, so this is correct behaviour rather than a bug. Cron simply cannot express the two conditions together in one line.

There is a subtlety almost every home-made parser gets wrong: any value starting with * counts as unrestricted. So a step written in the day-of-month field leaves that field open, and the OR rule never applies. This page follows that rule exactly, which is why it stays quiet on expressions that look similar but are fine.

The same digit, a different day

Copy a line from a Linux tutorial into a Spring @Scheduled annotation and every run moves by one day. Nothing errors, nothing warns, and the job simply runs on the wrong day until somebody notices. Writing the day by name (MON) means the same thing on all three.

Daylight saving, the expensive one

Twice a year a local clock either skips an hour or repeats one. A job set for 02:30 can vanish that day; a job set for 01:30 can happen twice.

What a scheduler does about it varies by implementation, so this page tells you the date and the window rather than pretending to know the outcome. Debian cron runs a skipped job shortly after the jump and does not re-run a doubled one. Others skip and repeat. If a second run would double-charge someone, that date is worth knowing in advance.

What this tool does not do

• It does not run, schedule or monitor anything. There is no backend. It reads and predicts, nothing else.
• It does not know your server's timezone or cron implementation. It calculates against the zone you pick. A server set to the wrong timezone behaves differently, and no editor can see that from here.
• It does not predict run times for L, W or #. Those are Quartz extensions with their own rules. The page validates them and then says plainly that it will not guess, because a wrong time here is worse than no time.
• @reboot has no next run. It depends on when the machine starts, not on the calendar.
• It does not publish a page per expression. That is how the market leader owns thousands of long-tail searches, and it is exactly the thin, duplicated content this site refuses to make.

Is my expression private?

Yes. Parsing, the English reading, the next runs and every warning run in JavaScript already loaded in your browser. No network request carries your expression, nothing is logged or stored, and closing the tab clears it. Schedules are more revealing than they look: the timing of a backup, a billing run or a data export says a lot about how a system is built.