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Marvin's Toolbox.

systemd Timer Tester

Paste an OnCalendar expression to see its normalized form and its next run times in any time zone, just like systemd-analyze calendar. Convert it to and from cron, and generate matching .timer and .service units.

Runs entirely in your browser. Nothing you put in here gets uploaded.

Weekday, date, time and zone, each of them optional. Pasting a whole OnCalendar= line works too.

Normalized form

Mon..Fri *-*-* 09:00:00

WeekdayYearMonthDayHourMinuteSecond

In plain words

At 09:00 on Monday through Friday

Next run
…
Typical gap
…
needs at least two upcoming runs

Field by field

Mon..FriWeekday

Only on Monday through Friday

*Year

Every year

*Month

Every month

*Day

Every day that passes the weekday check

09Hour

Hour 9

00Minute

Minute 0

00Second

Second 0

Upcoming runs

Working out the next runs…

Cron

Cron line, same schedule

0 9 * * 1-5

Nothing was lost, this line fires at the same times.

Unit files

Leave empty to run as root.

Spreads the start over this span, so a fleet does not fire at once.

How late systemd may fire to batch wakeups. The default is 1min.

Catch up on a run missed while the machine was off.
Wake the machine from suspend for the run.

Syntax

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About the systemd Timer Tester Tool

This tool tests a systemd OnCalendar expression the way systemd-analyze calendar does. Paste something like Mon..Fri 09:00 and you get the normalized form exactly as systemd prints it, the meaning in plain English, a field by field breakdown and the next runs with a relative readout such as in 3 hours. Every field keeps one color from the characters you type to the words in the summary.

The parser and the elapse calculation are ported line by line from systemd's own calendarspec.c, so the tool accepts and rejects what systemd does, and its run times hold up across daylight saving changes and leap years. It also converts schedules to and from cron, and writes a matching .timer and .service pair.

What you can do

  • Normalize an OnCalendar expression without a Linux machine at hand
  • List the next runs of a systemd timer in any time zone
  • Check what a timer does around daylight saving changes
  • Find out why a calendar expression never fires
  • Convert a crontab line to a systemd timer
  • Convert a systemd timer to cron and see what cannot carry over
  • Generate a .timer and .service unit pair with Persistent, RandomizedDelaySec, AccuracySec and WakeSystem
  • Fix syntax mistakes with errors that name the field and the value

How to use the systemd Timer Tester

  1. 1Type or paste a calendar expression, a whole OnCalendar= line, or pick one of the examples.
  2. 2Compare the normalized form and the plain English sentence with what you meant.
  3. 3Pick the time zone of the machine and read through the upcoming runs.
  4. 4Convert the schedule to cron if you need it there, or paste a crontab line to go the other way.
  5. 5Fill in the unit settings, then copy or download the .timer and .service files.

How a calendar expression is built

The full form is weekday year-month-day hour:minute:second zone, and every part is optional. A missing date means *-*-*, a missing time means 00:00:00, and missing seconds mean :00. So Mon 12:00 normalizes to Mon *-*-* 12:00:00.

Each date and time field takes *, a value, a comma list, a range with two dots like 9..17, or a repetition like 0/15, which means that value and then every 15 after it. Weekdays take names and ranges like Mon..Fri, seconds may hold fractions, and the zone at the end is UTC or any IANA name like Europe/Berlin. A ~ in place of the last dash counts from the end of the month, so *-02~03 is the third last day of February and Mon *-05~07/1 is the last Monday in May.

Words like minutely, hourly, daily, weekly, monthly, quarterly, semiannually and yearly are shorthands for full expressions.

Daylight saving and impossible dates

systemd matches the wall clock of the zone, so a time skipped by the spring clock change does not fire that day, and a time repeated in autumn fires once. Rows where the clocks change between two runs carry a marker.

A date like *-02-30 is valid syntax that can never come true, and Thu 2027-01-01 asks for a Thursday on a Friday. The tool tells these apart from expressions whose dates have simply passed, and it warns about the rare expressions on which systemd's search gives up.

Timers compared with cron

Cron has no seconds, no years, no month-end days and no zone per schedule, so the converter flags each of those instead of dropping them quietly. The bigger trap is the day fields: with both a day and a weekday, cron fires when either one matches, while systemd requires both. Going from cron to systemd, such a line becomes two OnCalendar= lines on the same timer.

To explain a cron line in detail, use the Cron Expression Explainer, and to assemble one field by field, use the Cron Expression Builder.

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