Cron Expression Parser

A cron expression is five fields read left to right: minute, hour, day of month, month and day of week. Each field says which values of that unit should trigger the job, and the job runs at every instant where all five match at once. It is a compact language, and like most compact languages it is easy to write something that means something other than what you intended — which is exactly why a cron expression parser is worth more than a cheat-sheet.

This cron parser does three things with whatever you type: it renders the schedule as a plain-English sentence, it breaks each field out so you can confirm it expands the way you meant, and it projects the next run times in your device's timezone so you can see the schedule as you would experience it. Everything parses as you type, in the browser.

The five fields of a cron expression

Months and weekdays also accept three-letter names, so mon-fri reads exactly the same as 1-5 and is far easier to review.

The operators in cron syntax

Every field is built from a small set of pieces that combine freely:

• an asterisk matches every value in range, so * * * * * fires every minute;
• a single number picks one value, so 0 in the minute field means the top of the hour;
• a hyphen is a range, as in 9-17 for the working day;
• a slash is a step, as in */5 for every fifth value;
• a comma joins a list of any of these, as in 0,30 for the top and bottom of the hour.

Putting that together, 0 9 * * 1-5 means minute 0 of hour 9, on every day of every month, but only Monday to Friday — nine in the morning on weekdays.

The day-of-month and day-of-week rule

When both the day-of-month and the day-of-week fields are set to something other than an asterisk, most cron implementations fire if EITHER one matches, not when both do. So 0 0 1 * 1 runs on the first of the month and on every Monday, which surprises almost everyone the first time they meet it. Leave one of the two fields as an asterisk whenever you mean a specific kind of day.

Timezone is the quiet bug

Cron carries no timezone; it runs in whatever zone the scheduler is configured for, which on a server is very often UTC. A job written as 0 9 * * * fires at nine in the morning server time, which might be the middle of the night where you are. This tool projects the next run times in your device's local timezone so you can see the schedule as you would experience it, but always confirm which zone your scheduler actually uses before you rely on it.

Beyond five fields

Schedulers such as Quartz and Spring use six or seven fields, adding a leading seconds position and sometimes a trailing year. That is a different dialect with its own rules. This parser targets the standard five-field crontab format used by Unix cron and the cron field of most Kubernetes CronJob definitions.

What a cron expression parser is for

Reviewing someone else's scheduled job is the canonical use: the string sits in a config file, everyone assumes it does what its comment says, and nobody has verified since it was written. Pasting it into a cron parser converts the assumption into a sentence and a list of upcoming fire times. Writing a new schedule is the second use — you get instant feedback on what */5 in the hour field really means, before the job is deployed somewhere that runs in UTC and fires at what would be the middle of your night.

Common cron expressions and what they mean

A cron checker like this table is also the fastest way to author a new expression: find the closest row, adjust one field, and read the plain-English description until it says what you meant.

A free cron parser that runs locally

Schedules frequently encode business logic — report timings, billing windows, maintenance slots — and live alongside job names that are internal. This cron expression parser runs entirely in your browser: nothing is uploaded, nothing is logged, and the next-run projection uses your device's clock rather than a server's. When you need to go the other way — building a schedule from a sentence — start from the presets and edit one field at a time, checking the plain-English description after each change; it is the same incremental discipline that makes regex and cron both tractable.