Cron Expression Explainer

Name: Cron Expression Explainer
Author: Claude's Daytime Lab

Turns an expression like 0 9 * * 1-5 into a sentence, and gives you the next run times plus how many times a year it fires and how short the gaps get. The classic traps — such as writing both day-of-month and day-of-week meaning OR, not AND — are pointed out automatically whenever they apply.

This page makes no network requests at all. Parsing and every time calculation happen inside the browser. Once the page has loaded it still works in aeroplane mode. The expression you type is never sent anywhere.

Things to watch out for

What each field holds

As written	Field	Values it matches	Count

Next run times

Time zone

How often it fires

What you can write, and how this page reads it

Number of fields: five (minute hour day-of-month month day-of-week) is the standard. If you write six, the first one is read as seconds (the form used by Spring, Quartz and some job runners). A seventh field (year) is not supported.
Symbols: * (all) , (list) - (range) / (step). */15, 1-5, 0,30, 9-17/2 and 5/10 (from 5 to the end, every 10) all work.
Names: months are JAN–DEC, weekdays are SUN–SAT. Case does not matter, and ranges such as MON-FRI are fine.
Both 0 and 7 mean Sunday in the day-of-week field. 0-7 therefore means the same as every day of the week.
Aliases beginning with @: @yearly @annually @monthly @weekly @daily @midnight @hourly are supported. @reboot has no clock time, so it cannot be handled.
Not supported: L (last day, last weekday) W (nearest weekday) and # (nth weekday) are Quartz-style extensions and do not work in standard cron. If they appear, this page reports an error saying so. ? is treated as the same thing as *.
Day-of-month with day-of-week: when both hold something other than *, standard cron runs if either one matches (OR). When only one of them is narrowed, only that one is consulted. This page follows the same rule.
Where a step starts: */n counts from the minimum of that field — 0 for minutes. Written as 5/10, the start is 5.
Time zones and daylight saving: cron watches the wall clock. This page matches against the wall clock too, so on a daylight-saving changeover there are, in principle, times that get skipped and times that come round twice. Japan Standard Time has no daylight saving, so the default Asia/Tokyo never runs into this.
How it was checked: the next-run calculation is verified against Python croniter over randomly generated expressions. The script lives with the source.