About

A misconfigured cron expression can silently skip critical backups, flood a server with requests every second instead of every hour, or trigger billing jobs on wrong dates. The standard 5-field cron syntax (minute hour day-of-month month day-of-week) is compact but error-prone. Field ranges differ: minutes span 0 - 59, hours 0 - 23, days 1 - 31, months 1 - 12, weekdays 0 - 6 (Sunday = 0). This tool constructs valid expressions through a visual interface and immediately shows the next 10 execution timestamps so you can verify behavior before deployment.

Note: this generator follows POSIX crontab semantics. When both day-of-month and day-of-week are specified (not *), the job runs if either field matches - a common source of confusion. Non-standard extensions like @reboot or seconds fields are outside scope. Execution times are calculated in your local timezone and assume the host clock is synchronized.

Formulas

A standard cron expression consists of 5 space-separated fields:

minute hour day-of-month month day-of-week

Each field accepts the following notation:

* → every possible value in range

n → exact value (e.g., 5)

a,b,c → list of specific values

a-b → range from a to b inclusive

*/n → every n-th value starting from range minimum

a-b/n → every n-th value within range a - b

Where field ranges are: minute = 0 - 59, hour = 0 - 23, day-of-month = 1 - 31, month = 1 - 12, day-of-week = 0 - 6 (Sunday = 0). POSIX specifies that when both day-of-month and day-of-week are restricted (not *), the job fires when either condition is met (logical ∨).

Reference Data

Expression	Description	Use Case
* * * * *	Every minute	Health checks, queue polling
/5 * * *	Every 5 minutes	Metrics collection
0 * * * *	Every hour at minute 0	Hourly cache flush
0 /2 * *	Every 2 hours	Data sync
0 0 * * *	Daily at midnight	Log rotation, backups
0 6 * * *	Daily at 06:00	Morning report emails
30 4 * * *	Daily at 04:30	Off-peak DB maintenance
0 0 * * 0	Every Sunday at midnight	Weekly digest
0 0 * * 1-5	Weekdays at midnight	Business-day processing
0 0 1 * *	1st of every month at midnight	Monthly invoicing
0 0 1 1 *	January 1st at midnight	Annual license renewal
0 0 15 * *	15th of every month	Mid-month payroll
0 9-17 * * 1-5	Hourly 09:00-17:00, Mon - Fri	Business-hours monitoring
/15 * * *	Every 15 minutes	API polling
0 0 1,15 * *	1st and 15th of month	Bi-monthly billing
0 22 * * 5	Every Friday at 22:00	Weekend deploy trigger
5 4 * * 0	Sunday at 04:05	Weekly full backup
0 0 * * 6,0	Weekends at midnight	Weekend batch jobs
0 /6 * *	Every 6 hours	Certificate checks
0 0 L * *	Last day of month (non-standard)	End-of-month reports

Frequently Asked Questions

Per POSIX crontab specification, if both fields are restricted (neither is *), the job runs when either condition matches. For example, 0 0 15 * 5 fires at midnight on every 15th of the month AND every Friday, not only Fridays that fall on the 15th. This OR-logic is a frequent source of unexpected extra executions.

In standard POSIX cron, 0 is Sunday and 6 is Saturday. Some implementations also accept 7 as Sunday for convenience. This generator uses the 0 - 6 convention where 0 = Sunday, 1 = Monday.

This tool calculates next execution times in your browser's local timezone. Server-side cron typically runs in the server's configured timezone (often UTC). If your server is set to UTC and you are in UTC+5, there will be a 5-hour offset. Always verify the timezone setting in /etc/crontab or the TZ environment variable.

Cron silently skips impossible dates. Specifying day 31 will not fire in February, April, June, September, or November. If you need end-of-month execution, the standard 5-field cron cannot express "last day of month" natively. You would need wrapper scripts or non-standard extensions like the L flag in some implementations.

Functionally they are identical: both fire at every minute divisible by 5. The */5 syntax is shorthand. However, */n always starts from the field minimum (0 for minutes). If you need an offset (e.g., minutes 3, 8, 13...), use 3-59/5 instead.

The expression 10-30/5 in the minute field means "every 5th minute within the range 10 to 30", yielding minutes 10, 15, 20, 25, 30. The step starts from the range's lower bound, not from 0. If the step exceeds the range width, only the start value fires.

Cron does not wait for the previous invocation to finish. If a job scheduled every minute takes 90 seconds, two instances will overlap. Use file-lock mechanisms (flock) or check for running PIDs in your script. This generator shows execution frequency so you can assess overlap risk before deploying.