Free Cron Expression Generator – Build & Explain Cron

How Cron Expressions Work and How to Build One

Cron is the scheduling language Unix has used for decades, and it is compact to the point of being cryptic. Five fields separated by spaces describe when a job should run, and a single wrong character turns a nightly backup into a job that fires every minute. This generator lets you pick each field from a dropdown, shows the assembled expression, and states in plain English exactly when it will fire. Sysadmins writing a crontab, developers configuring a scheduled GitHub Action, and platform engineers reviewing an inherited job list all use it for the same reason: reading cron is much harder than writing it with help. The five fields are, in order, minute from 0 to 59, hour from 0 to 23, day of month from 1 to 31, month from 1 to 12, and day of week from 0 to 6 with 0 meaning Sunday. Four symbols do all the work. An asterisk means every possible value for that field. A slash introduces a step, so the pattern star slash 15 in the minute field means every fifteenth minute. A hyphen makes an inclusive range, so 9-17 in the hour field covers nine in the morning to five in the afternoon. A comma lists specific values, as in 1,15 for the first and fifteenth. The rule that surprises people is how day of month and day of week combine: when both are set to something other than an asterisk, most cron implementations treat them as an OR rather than an AND, so the job runs on either condition. Build one. Suppose you want a database backup at 2:30 every morning. Set minute to 30, hour to 2, and leave day of month, month and day of week as asterisks. The expression is 30 2 star star star and the description reads at 02:30 every day. Now change it to weekdays only by setting day of week to 1-5, giving 30 2 star star 1-5, which fires Monday through Friday at the same time. For a different shape entirely, take star slash 10 9-17 star star 1-5: that runs every ten minutes between nine in the morning and half past five in the afternoon, Monday to Friday, which comes to 54 executions per working day. Reading that off the raw expression is hard; reading it off a sentence is not. Where it gets used. A sysadmin schedules a log rotation at 3:15 on the first of each month. A developer configures a nightly test run in a CI pipeline that accepts cron syntax. A data engineer sets a warehouse sync to run every fifteen minutes during business hours only, to keep query costs down. A team lead documents an inherited crontab by pasting each expression in and writing the plain-English description beside it in the runbook, which is often the first time anyone has understood what those jobs actually do. A developer schedules a cache warm-up for ten minutes before the daily traffic peak. Two things to check before you trust a schedule. First, timezone: cron runs in the timezone of the machine or the scheduler, not yours, so an expression that reads 02:30 fires at 02:30 server time, and a container defaulting to UTC will run your job five and a half hours earlier than an Indian office expects. Confirm the timezone explicitly. Second, the standard format here is the five-field one used by Linux crontab and most CI systems, but Quartz and some cloud schedulers add a seconds field at the front or a year field at the end, and AWS EventBridge uses question marks in the day fields; check your platform documentation before pasting. Everything runs in your browser and nothing is uploaded.

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