Cron Expression Parser

How cron expressions work

Cron is the scheduling syntax behind almost every Unix-like system, CI pipeline, and serverless scheduler. A single line of five space-separated fields tells the scheduler exactly when to run a job. The syntax is terse and easy to misread, which is why a parser that turns it back into plain English is so useful.

The five fields

A standard cron expression has five fields, always in this order:

┌───────────── minute        (0 - 59)
│ ┌───────────── hour         (0 - 23)
│ │ ┌───────────── day-of-month (1 - 31)
│ │ │ ┌───────────── month      (1 - 12)
│ │ │ │ ┌───────────── day-of-week (0 - 6, Sun=0)
│ │ │ │ │
* * * * *

Every field accepts * to mean "any value". A bare * in all five fields runs the job once a minute, forever.

Operators inside a field

• * — every valid value for that field.
• 5 — a single literal value.
• 1-5 — an inclusive range (Monday through Friday in the day-of-week field).
• */10 — a step: every 10th value starting at the field minimum.
• 0,30 — a list of specific values.

Common patterns

*/5 * * * *     every 5 minutes
0 * * * *       at the top of every hour
0 9 * * *       every day at 09:00
0 9 * * 1-5     weekdays at 09:00
0 0 1 * *       midnight on the 1st of each month
30 2 * * 0      02:30 every Sunday

The day-of-month / day-of-week gotcha

When both the day-of-month and day-of-week fields are restricted, POSIX cron runs the job if eitherone matches. The expression 0 0 13 * 5 therefore fires on the 13th of every month andon every Friday — not only on Friday the 13th. Leave one of the two fields as * to avoid the surprise.

Timezones and resolution

Cron evaluates in the timezone of the daemon or platform — commonly UTC in cloud environments. The smallest interval it can express is one minute; sub-minute scheduling needs an extended scheduler with a seconds field. Always confirm the timezone your runner uses before trusting a daily or hourly schedule in production.

Related dev tools

• JSON Formatter — tidy the config or job definition your cron lives in.
• Regex Tester — test patterns used alongside scheduled scripts.
• UUID Generator — generate unique IDs for scheduled job runs.
• Time Converter — convert intervals between seconds, minutes, hours, and days.