Cron expression explainer with next runs, time zones and DST warnings

What it does

Check what a cron schedule really means and when it will actually run.

• Supports standard 5-field cron, 6-field with seconds (Spring, Quartz, node-cron style) and aliases such as @daily
• Shows each field's expanded values and a plain-English description
• Lists the next N runs in local time and UTC for any IANA time zone or a fixed offset such as +09:00
• Warns about the day-of-month/day-of-week OR rule, star-step fields like */2, months without day 31, Feb 29, and DST gaps and repeated hours
• Rejects out-of-range values, reversed ranges and Quartz-only L W # with a clear reason

How to use

python scripts/cron_explain.py "*/15 9-17 * * MON-FRI" --tz Asia/Tokyo --count 10
python scripts/cron_explain.py "30 2 * * *" --tz America/New_York --from 2026-03-07T00:00
python scripts/cron_explain.py @weekly --json

Ask Claude, Codex, Hermes Agent or Cursor to explain or write a cron schedule; it confirms the dialect and time zone, verifies with the script, and delivers the final expression with its next runs.

Requirements

• Python 3.10+ (standard library zoneinfo)
• If a zone is not found (common on Windows), run python -m pip install tzdata or pass a fixed offset

Caveats

• It only parses and prints; it never edits crontabs or scheduler settings.
• DST handling and syntax such as 5/15 differ between implementations. Calculations follow Vixie cron / cronie, and possible differences are flagged as warnings.
• Quartz L W # and the 7th year field are not supported.