Free Cron Expression Builder

How It Works

The cron expression builder runs entirely in your browser using a hand-written parser in lib/cron.ts — no external library, no server call. It validates the expression, assembles a plain-English description using the browser's built-in Intl.DateTimeFormat and Intl.ListFormat APIs for locale-aware day and month names, and computes the next scheduled run times by walking forward through time with field-skipping logic rather than brute-forcing every minute.

1. 1
   Expression parsed field-by-field

   The parser splits the expression into five (or six) whitespace-delimited fields and tokenizes each one for commas, ranges (e.g. 1-5), step notation (e.g. */15), and named values like MON or JAN. @macros such as @daily are expanded to their five-field equivalents before parsing begins. Invalid values or out-of-range numbers surface as typed error keys that map to localized messages.
2. 2
   Human-readable description assembled

   describeCron() in lib/cron.ts builds the English (or localized) sentence by inspecting which fields are wildcards versus explicit sets. Day and month names are resolved through Intl.DateTimeFormat with the active locale, and list items are joined using Intl.ListFormat so the conjunction ('and' vs ',' vs locale-specific form) matches the user's language.
3. 3
   Next run times computed with field skipping

   nextRuns() walks a Date object forward from the current moment, but skips entire months, days, or hours that cannot match rather than testing every minute. This means an impossible schedule like Feb 30 exits quickly through a five-year horizon check rather than looping 2.6 million times.
4. 4
   Expression shared via the URL fragment

   Any expression you type is written to the page's URL fragment as #expr= (debounced 500 ms via window.history.replaceState). The fragment is never sent to a server; the page hydrates from it on load, so sharing or bookmarking a URL restores the exact expression without any server state.

Related Tools