Crontab Guru

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Build me a cron expression editor: type a schedule, read it back in English, and see the next runs.

This is small, and it is the kind of small that goes wrong quietly, so the whole prompt is about correctness.

STACK
- Node 20+, or nothing at all — this works entirely in a browser as one static page
- A real time zone library and the IANA database. Never an offset
- No framework, no build step

PARSING, WHICH IS THE WHOLE JOB
- Five fields: minute, hour, day of month, month, day of week. Support the extras people actually use: `*/5`, ranges, lists, `L`, `#`, and three-letter month and day names
- **Day-of-month and day-of-week together are OR, not AND.** `0 0 13 * 5` fires on the 13th *and* on every Friday, not on Friday the 13th. That single rule is the most misunderstood thing in cron, and getting it wrong makes the tool worse than useless
- Both fields unrestricted (`*`) means every day. One restricted and the other `*` means only the restricted one applies. Both restricted means either matches
- Support the named shorthands — `@daily`, `@weekly`, `@reboot` — and say plainly that `@reboot` has no next run to show
- Reject rather than guess: `0 0 31 2 *` never fires, and the tool should say so rather than silently showing nothing

SAYING IT IN ENGLISH
- Build the sentence from the parsed fields, not from string matching on the input
- Read it aloud before shipping each shape: 'every 5 minutes', 'at 09:30 on weekdays', 'at midnight on the 1st of every month'
- When the expression is complicated enough that no sentence is clear, say the parts separately rather than producing a sentence nobody can follow

THE NEXT RUNS, WHERE THE REAL BUGS ARE
- Show the next several occurrences as actual dates and times, in a zone the reader picks
- **Daylight saving is the part that bites.** In the spring transition, 02:30 does not exist on one day of the year — a job scheduled then either skips or runs at a different hour, and which one depends on the cron implementation. In the autumn transition it happens twice
- Say what your calculation assumes, out loud, on the page. Different cron daemons genuinely behave differently here and a tool that pretends there is one answer is lying
- Compute against the chosen zone with a real library, never by adding hours to a UTC instant
- A test suite with both transitions, a leap year, the 29th of February, and the day-of-week OR case. Those five cover almost every real bug in this category

THE PAGE
- One input, the sentence beneath it, the next runs beneath that, updating as you type
- A list of common examples to click, which is how most people actually use it
- The URL carries the expression, so a schedule can be shared or linked from a review comment
- Works with no network after the first load, because it is a parser and a calendar

AND THE HALF THAT IS ACTUALLY WORTH PAYING FOR
The editor is free everywhere; the subscription is the monitoring dashboard beside it — knowing that the job did not run. That is a different product and it is under Healthchecks and Cronitor elsewhere in this catalogue.

WHAT MATTERS MOST
The day-of-month OR day-of-week rule, and the daylight-saving cases. Write those tests first; everything else here is a form.