Cron Expression Generator

A cron expression is a line of five fields that tells a scheduler when to run a job: minute, hour, day of month, month and day of week, in that order. `0 9 * * 1-5` means nine in the morning, Monday through Friday. The asterisks mean “every value”, so a field you do not care about is left as one.

Start from the frequency and fill in only what you need. Decide the minute, then the hour, then whether it repeats daily, weekly, monthly or yearly, and leave the rest as asterisks. The builder above does this for you and shows the result as you go — pick a schedule on the left and the expression appears on the right, with a sentence explaining exactly what it will do.

Minute (0–59), hour (0–23), day of month (1–31), month (1–12) and day of week (0–7). They are always in that order and always separated by spaces. Sunday can be written as either 0 or 7 — both are accepted by standard cron and this tool treats them identically.

An asterisk means every value the field allows. In the minute field it means all sixty minutes; in the month field it means all twelve months. `* * * * *` is therefore every minute of every hour of every day — the busiest schedule you can write with five fields.

It is a step: every fifth value, counting from the start of the field. In the minute field `*/5` fires at :00, :05, :10 and so on — twelve times an hour. The slash always means “every nth”, and what it steps through is whatever sits in front of it, so `9-17/2` means every second hour between 9 and 17.

Use `*/5 * * * *`. That fires at twelve points in every hour, all day, every day. For every 10, 15 or 30 minutes, change the number: `*/10`, `*/15`, `*/30`. Intervals that divide 60 evenly keep the gaps equal; `*/7` would fire at :00, :07 … :56 and then wait only four minutes for the next hour to start.

Use `0 * * * *` — at minute zero of every hour. Do not use `* * * * *`, which runs every minute, or `0 0 * * *`, which runs once a day at midnight. If you want to avoid the top of the hour, when every other job on the machine is also starting, use a different minute: `17 * * * *` is a perfectly ordinary choice.

Put the minute and hour in the first two fields and leave the rest as asterisks. `0 9 * * *` is 9:00 AM daily, `30 17 * * *` is 5:30 PM daily, and `0 0 * * *` is midnight. Remember that the hour field uses a 24-hour clock, so 5 PM is 17.

Set the day-of-week field to `1-5`, which is Monday through Friday. `0 9 * * 1-5` runs at nine every working day and skips the weekend. For weekends only, use `0,6` — Sunday and Saturday. Leave day of month as an asterisk, or you will trigger cron's OR rule and the job will run on far more days than you intended.

Use `0 0 1 * *` — midnight on the 1st, every month. Change the hour and minute for a different time, and the third field for a different day. Avoid days 29 to 31 if the job needs to run every month: the 31st exists in only seven months, so a “monthly” job on the 31st actually runs seven times a year.

They are separate fields and, crucially, they are combined with OR rather than AND. If both are set to something other than an asterisk, the job runs when either matches. `0 0 13 * 5` runs on the 13th of every month *and* on every Friday — not on Friday the 13th. If only one of the two is restricted, only that one applies. This is the single most misunderstood rule in cron, and the tool above flags it whenever your expression triggers it.

Not with a cron expression alone. Because the two day fields are OR-ed rather than AND-ed, there is no way to say “the 13th, but only when it is a Friday”. The usual answer is to schedule the job for every 13th with `0 0 13 * *` and have the script itself exit early when the weekday is not Friday.

The local time zone of the machine the scheduler runs on, which on most servers and in most containers is UTC. That is why a job set for 9 AM from a laptop in London or Mumbai may fire at a quite different local hour in production. Some implementations let you set `CRON_TZ` at the top of the crontab or a `TZ` variable for the job. The next-run preview above lets you pick the zone so you can check both.

It depends on the implementation, and this is one of the few places where there is no single right answer. When the clocks go forward, a time like 02:30 does not exist that day — some versions of cron run the job anyway, some skip it. When the clocks go back, 02:30 happens twice, and some versions run the job twice. Jobs scheduled outside the 01:00–03:00 window are unaffected. The preview above marks any run that lands in one of these windows rather than quietly picking an answer.

Standard Unix cron has five. If you are looking at six or seven, it is almost certainly a Quartz or Spring expression, which adds a seconds field at the front and an optional year at the end. Pasting a six-field expression into a Linux crontab shifts every field one place and produces a schedule nobody intended — this tool reports that case rather than trying to interpret it.

Quartz is a Java scheduling library with its own dialect. It uses six or seven fields rather than five, numbers the days of the week 1–7 with Sunday as 1 rather than 0–7 with Sunday as 0 or 7, requires a `?` in one of the two day fields, and adds `L`, `W` and `#` for things like “last Friday of the month”. The two formats look similar and are not interchangeable. This tool generates standard Unix cron only.

No, and that is deliberate. Standard Unix cron has no seconds field — the smallest interval it can express is one minute. Tools that offer a seconds field are generating Quartz or a systemd timer, and pasting that output into a crontab produces a broken schedule. If you need sub-minute work, run a job every minute that loops internally, or use a systemd timer or a task queue instead.

They are convenience shortcuts that Vixie cron accepts: `@hourly` is `0 * * * *`, `@daily` and `@midnight` are `0 0 * * *`, `@weekly` is `0 0 * * 0`, `@monthly` is `0 0 1 * *`, `@yearly` and `@annually` are `0 0 1 1 *`, and `@reboot` runs once at startup. They are not part of POSIX, and container schedulers and managed cron services often reject them, so this generator emits the five-field form — which means the same thing and works everywhere.

`crontab -l` lists the current user's jobs, `crontab -e` opens them in an editor, and `crontab -r` deletes all of them — that last one has no confirmation prompt, so it is worth keeping a copy. System-wide jobs live in `/etc/crontab` and `/etc/cron.d/`, which have an extra field for the user the job runs as.

In rough order of likelihood: the expression matches a different time than you think — paste it into the tool above and read the sentence; the server is in a different time zone; the job's environment has a minimal `PATH` and cannot find the command, so absolute paths are safer; the script is not executable; the crontab has no trailing newline, which some versions require; or the command contains an unescaped `%`, which cron treats as end-of-command. Redirecting output to a file with `>> /tmp/job.log 2>&1` usually answers the question in one run.

Yes. Cron starts a job whenever the schedule matches, whether or not the previous run has finished, so a job that takes twelve minutes on a five-minute schedule will end up with three copies running at once. If that is a problem, wrap the command in `flock` — `flock -n /tmp/job.lock command` — which quietly skips a run rather than piling up.

Read it rather than run it. Paste the expression into the tool above: the sentence tells you what it does, the field breakdown shows exactly which values each field matches, and the next-run preview lists the actual dates and times it will fire in the zone you choose. That catches the vast majority of mistakes in seconds, which no amount of waiting for the real thing will.

The most frequent is once a minute — `* * * * *` — because minutes are the finest unit the five-field format has. There is no upper limit on the gap: `0 0 1 1 *` runs once a year, and `0 0 29 2 *` runs on 29 February, which means once every four years. The tool above will tell you when an expression can never run at all, such as one scheduled for 30 February.

It is free and nothing leaves your device. The parser, the description, the validation and the next-run search all run in your browser as JavaScript — there is no server call and no analytics on what you type. You can confirm it by opening your browser's network panel, or by disconnecting from the internet: the page carries on working.