# Cron expression parser and explainer

> Translate a five-field cron expression or crontab line into plain English and list its next run times after any start date and time, in UTC.

Interactive version: https://www.calcopenly.com/programming/cron-expression-parser
Subject: Programming and tech calculators

A cron expression has five fields: minute (0–59), hour (0–23), day of month (1–31), month (1–12 or JAN–DEC) and day of week (0–7 or SUN–SAT, where 0 and 7 both mean Sunday). Each field takes a single value, a comma-separated list, a range with a hyphen or a step with a slash. The parser expands every field into its set of values, then scans forward day by day for the minutes that match all five.

The default, */15 9-17 * * MON-FRI, runs every 15 minutes from 09:00 to 17:45 on weekdays, which is 36 runs on each matching day. Started at midnight on Thursday 15 January 2026, its first run is 09:00 that morning.

When day of month and day of week are both restricted, cron runs on days that match either one, as crontab(5) specifies. Times here are UTC; a cron daemon uses its server's time zone, or CRON_TZ in cronie.

## Inputs

- **Cron expression**: minute hour day-of-month month day-of-week, e.g. 30 2 * * 1 — or @hourly, @daily, @weekly, @monthly, @yearly.
- **Start date (UTC)**
- **Start time (UTC)**: Runs are listed strictly after this moment.
- **Runs to list**

## Results

- Next run — main result
- In plain English
- Upcoming runs
- Time until the next run
- Runs on each matching day

## Formula

$$
\begin{aligned} \text{run} &\iff m \in M \land h \in H \\ &\qquad \land\ \text{month} \in Mo \land \text{day} \\ \text{day} &= \begin{cases} \text{DOM} \lor \text{DOW} & \text{both} \ne * \\ \text{DOM} \land \text{DOW} & \text{otherwise} \end{cases} \end{aligned}
$$

## Worked examples

### Every 15 minutes in office hours

- Cron expression: */15 9-17 * * MON-FRI
- Start date (UTC): 2026-01-15
- Start time (UTC): 00:00
- **Next run: Thu 2026-01-15 09:00 UTC**
- **Upcoming runs: Thu 2026-01-15 09:00 UTC
Thu 2026-01-15 09:15 UTC
Thu 2026-01-15 09:30 UTC
Thu 2026-01-15 09:45 UTC
Thu 2026-01-15 10:00 UTC
Thu 2026-01-15 10:15 UTC
Thu 2026-01-15 10:30 UTC
Thu 2026-01-15 10:45 UTC
Thu 2026-01-15 11:00 UTC
Thu 2026-01-15 11:15 UTC**
- **Runs on each matching day: 36**
- **Time until the next run: 9 h**
- Checked against: Brute-force Python 3.8 datetime scan minute by minute with an independent field parser; 4 minutes × 9 hours = 36 runs a day; 09:00 − 00:00 = 32400 s

### Daily at 09:00, starting exactly at 09:00 (edge)

- Cron expression: 0 9 * * *
- Start date (UTC): 2026-01-15
- Start time (UTC): 09:00
- Runs to list: 3
- **Next run: Fri 2026-01-16 09:00 UTC**
- **In plain English: At 09:00, every day**
- **Upcoming runs: Fri 2026-01-16 09:00 UTC
Sat 2026-01-17 09:00 UTC
Sun 2026-01-18 09:00 UTC**
- Checked against: crontab(5): runs are strictly after the start; confirmed by the Python brute-force scan

### Leap day only

- Cron expression: 0 0 29 2 *
- Start date (UTC): 2026-01-15
- Start time (UTC): 00:00
- Runs to list: 3
- **Next run: Tue 2028-02-29 00:00 UTC**
- **Upcoming runs: Tue 2028-02-29 00:00 UTC
Sun 2032-02-29 00:00 UTC
Fri 2036-02-29 00:00 UTC**
- Checked against: Python 3.8 calendar.isleap and datetime weekday, via the brute-force scan

### Day 1 or Monday (both day fields restricted)

- Cron expression: 0 12 1 * MON
- Start date (UTC): 2026-01-15
- Start time (UTC): 00:00
- Runs to list: 5
- **Upcoming runs: Mon 2026-01-19 12:00 UTC
Mon 2026-01-26 12:00 UTC
Sun 2026-02-01 12:00 UTC
Mon 2026-02-02 12:00 UTC
Mon 2026-02-09 12:00 UTC**
- Checked against: crontab(5) OR rule for day-of-month and day-of-week, checked by the Python brute-force scan

### Star-step day of month uses AND (Vixie quirk)

- Cron expression: 0 0 */2 * FRI
- Start date (UTC): 2026-01-15
- Start time (UTC): 00:00
- Runs to list: 3
- **Upcoming runs: Fri 2026-01-23 00:00 UTC
Fri 2026-02-13 00:00 UTC
Fri 2026-02-27 00:00 UTC**
- Checked against: Vixie cron entry.c sets DOM_STAR when the field starts with *, so both fields must match; Python brute-force scan

### @weekly macro

- Cron expression: @weekly
- Start date (UTC): 2026-01-15
- Start time (UTC): 00:00
- Runs to list: 2
- **Next run: Sun 2026-01-18 00:00 UTC**
- **Upcoming runs: Sun 2026-01-18 00:00 UTC
Sun 2026-01-25 00:00 UTC**
- Checked against: crontab(5): @weekly = 0 0 * * 0; Python brute-force scan

## Questions

### What does */5 mean in a cron expression?

*/5 means every 5th value of the field, counted from the field's first value. In the minute field it fires at minutes 0, 5, 10 … 55, which is 12 times an hour. A step can follow a range too: 10-30/10 gives minutes 10, 20 and 30. Steps restart each hour, so */7 runs at minute 56 and then again at minute 0.

### How do I run a cron job every day at midnight?

Write 0 0 * * *, which means minute 0 of hour 0 on every day, or use the @daily macro, which cronie and Vixie cron expand to the same five fields. For 2:30 a.m. every day write 30 2 * * *, and for 09:00 on weekdays write 0 9 * * 1-5. The hour is read in the server's time zone unless the crontab sets CRON_TZ.

### How do day of month and day of week combine in cron?

If both fields are restricted, cron runs when either one matches, so 0 12 1 * MON fires at noon on the 1st of every month and on every Monday. In Vixie cron and cronie, a day field that starts with * switches this to both, which is why 0 0 */2 * FRI runs only on Fridays that fall on odd dates such as the 23rd.

### Why does my cron expression have 6 or 7 fields?

Standard cron uses 5 fields. Quartz and Spring schedulers put a seconds field in front, making 6, and Quartz accepts an optional year as a 7th, so an expression like 0 0/15 * * * ? belongs to those tools rather than crontab; the ? placeholder is Quartz syntax. This parser reads the 5-field crontab format plus the @hourly, @daily, @weekly, @monthly and @yearly macros.

### Can cron run a job every 30 seconds?

Not directly: the smallest unit in a crontab is 1 minute, because the first field is the minute. The usual workaround is two entries, * * * * * job and * * * * * sleep 30; job, which start the job at 0 and 30 seconds past each minute. systemd timers accept OnCalendar schedules down to the second if you need finer control.

### How accurate is the cron expression parser and explainer?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 7 worked examples whose answers come from independent sources; for example, “Every 15 minutes in office hours” is checked against Brute-force Python 3.8 datetime scan minute by minute with an independent field parser; 4 minutes × 9 hours = 36 runs a day; 09:00 − 00:00 = 32400 s.

### Where does the method come from?

POSIX.1-2017 (IEEE Std 1003.1) — crontab utility, input file format; crontab(5) — cronie / Vixie cron: ranges, steps, names, @ macros and the day-of-month / day-of-week rule.

## Sources

- [POSIX.1-2017 (IEEE Std 1003.1) — crontab utility, input file format](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/crontab.html)
- [crontab(5) — cronie / Vixie cron: ranges, steps, names, @ macros and the day-of-month / day-of-week rule](https://man7.org/linux/man-pages/man5/crontab.5.html)