How to Read and Understand Cron Expressions

A cron expression is a string of five or six fields that describes a recurring schedule. Once you learn the pattern, you can read any cron expression like plain English. This guide teaches you to decode every cron syntax element, from simple * * * * * to complex expressions with ranges, steps, and special characters.

The Five Fields of Cron

Every cron expression has five time fields followed by the command:

┌───────── Minute (0-59)
│ ┌──────── Hour (0-23)
│ │ ┌─────── Day of Month (1-31)
│ │ │ ┌────── Month (1-12)
│ │ │ │ ┌───── Day of Week (0-7, 0 and 7 = Sunday)
│ │ │ │ │
* * * * * <command>

Reading from left to right: "At what minute, on what hour, on which day of the month, in which month, and on which day of the week?"

Reading Expressions Step by Step

Simple Examples

0 9 * * *

Read as: "At minute 0, at hour 9, every day of the month, every month, every day of the week." → Runs daily at 9:00 AM.

*/15 * * * *

Read as: "Every 15 minutes of every hour, every day." → Runs every 15 minutes.

30 8 * * 1-5

Read as: "At minute 30, at hour 8, every day of the month, every month, Monday through Friday." → Runs weekdays at 8:30 AM.

0 0 1 * *

Read as: "At midnight on the first day of every month." → Runs monthly.

Complex Examples

0 9-17/2 * * 1-5

Breaking it down:

• 0 — at minute 0
• 9-17/2 — every 2 hours from 9 to 17 (i.e., 9, 11, 13, 15, 17)
• * — every day of month
• * — every month
• 1-5 — Monday to Friday

→ Runs at 9:00, 11:00, 13:00, 15:00, and 17:00 on weekdays.

30 6 1,15 * 0

Breaking it down:

• 30 — at minute 30
• 6 — at hour 6
• 1,15 — on day 1 and day 15 of the month
• * — every month
• 0 — on Sunday only

→ Runs at 6:30 AM on the 1st and 15th of the month, only if those days are also Sundays.

Special Characters Reference

* — Every / All

Matches any value for that field.

0 * * * *     → "Every hour"
* * * * *     → "Every minute"

, — List / Or

Separates multiple values.

0 9,12,15 * * *     → "At 9 AM, 12 PM, and 3 PM"

- — Range

Defines an inclusive range of values.

0 9-17 * * *        → "Every hour from 9 AM to 5 PM"
30 8 * * 1-5        → "Weekdays at 8:30 AM"

/ — Step

Divides the field into increments.

*/15 * * * *        → "Every 15 minutes"
0 */2 * * *         → "Every 2 hours"
0 0 */3 * *        → "Every 3 days"

Steps can be combined with ranges:

0 9-17/2 * * *     → "Every 2 hours from 9 AM to 5 PM"
30 */4 * * 1-5     → "Every 4 hours on weekdays, at minute 30"

L — Last (some systems)

In some cron implementations (like Quartz), L means "last."

0 0 L * *          → "Last day of every month"

# — Nth occurrence (some systems)

0 9 * * 1#1        → "First Monday of every month at 9 AM"

Common Expressions Decoded

Six-Field Cron (With Seconds)

Some cron implementations (like Quartz Scheduler) use six fields, adding seconds at the beginning:

┌───────── Second (0-59)
│ ┌──────── Minute (0-59)
│ │ ┌─────── Hour (0-23)
│ │ │ ┌────── Day of Month (1-31)
│ │ │ │ ┌───── Month (1-12)
│ │ │ │ │ ┌──── Day of Week (0-7)
│ │ │ │ │ │
* * * * * * <command>

0 */5 * * * *     → "Every 5 minutes, at second 0"
0 0 9 * * *       → "Every day at 9:00:00 AM"

Non-Standard Shorthands

Many cron implementations support these aliases:

@yearly   → 0 0 1 1 *     → "Once a year"
@monthly  → 0 0 1 * *     → "Once a month"
@weekly   → 0 0 * * 0     → "Once a week"
@daily    → 0 0 * * *     → "Once a day"
@hourly   → 0 * * * *     → "Once an hour"
@reboot   → (not a time)   → "Run at startup"

Translating Cron to Human Readable

When you see an unfamiliar cron expression, decode it field by field:

Expression: 45 17 1,15 * 1-5

Result: "At 5:45 PM on the 1st and 15th of every month, only if those days are weekdays."

Expression: 0 0 * * 0

Result: "Every Sunday at midnight."

Common Pitfalls When Reading Cron

Pitfall 1: * in day-of-week AND day-of-month means both match

When both day-of-month and day-of-week are *, the expression runs on any day. But when both have specific values, the job runs if EITHER matches — it is an OR condition.

0 9 15 * 1

This runs at 9 AM on the 15th of the month AND on every Monday (not "the 15th AND Monday"). To run only when both match, you need two separate crontab entries or script-level logic.

Pitfall 2: Cron uses local time, not UTC

Cron uses the system's local time. On servers configured to UTC, 0 9 * * * runs at 9 AM UTC. Always verify the server timezone before setting schedules.

Pitfall 3: 0 and 7 are both Sunday

In the day-of-week field, both 0 and 7 represent Sunday. This is a legacy from early Unix implementations.

Online Tool

The Cron Expression Parser tool on Help2Code decodes any cron expression into a human-readable description. Paste an expression like */15 9-17 * * 1-5 and see a field-by-field breakdown, next run times, and a plain-English explanation. Build new expressions with the Cron Job Command Generator tool.

Conclusion

Reading cron expressions is a skill that becomes second nature with practice. Remember the five fields: minute, hour, day-of-month, month, day-of-week. Use the wildcard * for "every," ranges with - for spans, steps with / for intervals, and lists with , for specific values. Decode any unfamiliar expression field by field, and use the Cron Expression Parser tool to verify your understanding.