Cron Expression Validator & Next-Run Calculator

Find out what a cron expression actually does before you commit it to a scheduler. This Actor validates standard 5-field cron expressions, describes each one in plain English, and computes the next run times in UTC.

Pure algorithm, no third-party API, no signup.

What it does

For each expression you send, it:

1. Parses the five fields — minute, hour, day-of-month, month, day-of-week — supporting *, lists (1,2,3), ranges (1-5) and steps (*/15).
2. Reports whether the expression is syntactically valid.
3. Produces a plain-English description of the schedule.
4. Computes the next N run times in UTC (you choose N, from 1 to 20; the default is 5).

An invalid expression is reported as invalid in its own output row — it does not stop the rest of the batch.

Use cases

• Code review — paste the cron lines from a pull request and read back what they actually mean, instead of decoding */7 3 * * 2 by hand.
• Migrations and audits — dump every schedule from a crontab, a CI config or an ERP job table and confirm each one still means what its comment claims.
• Documentation — generate readable descriptions of your scheduled jobs for a runbook.
• Debugging a job that "never runs" — see the next computed run times, or discover the expression matches nothing at all.

Input

{
  "expressions": ["*/15 * * * *", "0 9 * * 1-5", "not a cron"],
  "runsToCompute": 5
}

expressions is required. runsToCompute is optional (default 5, minimum 1, maximum 20).

Output (one row per expression)

{
  "input": "0 9 * * 1-5",
  "valid": true,
  "description": "at minute 0 past hour 9, on day-of-week 1-5 (0=Sun)",
  "next_runs_utc": ["2026-08-31T09:00:00.000Z", "2026-09-01T09:00:00.000Z"]
}

Pricing

Pay-per-event: $0.003 per expression checked, charged once a completed answer is produced — valid or invalid.

Limitations — read this before you buy

• Standard 5-field cron only. No seconds field (the 6-field Quartz-style syntax is not parsed) and no @daily / @hourly / @reboot macros.
• UTC only. There is no timezone parameter. If your scheduler runs in a local timezone with daylight saving, the computed times will not match it.
• The next-run search is bounded to two years ahead. An expression that matches nothing in that window — 30 2 30 2 *, February 30th — is reported as syntactically valid with zero runs found, not as an error. That distinction is deliberate: the syntax really is valid, and silently calling it invalid would hide the actual problem.

FAQ

Why does my expression validate here but fail in my scheduler? Most likely a dialect difference: seconds fields and @macros are extensions that many schedulers accept and this Actor does not parse.

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