systemd Timer: OnCalendar Parser and Unit Generator

Features

• Parses *, a..b ranges, a/b steps, named weekdays and months, and ~ (back from month end)
• Computes the next 1–20 run times in the chosen time zone
• Shows each run as local time with its offset and as UTC
• Explains what the weekday, date and time segments each mean
• Generates a matching .timer and .service pair
• Accepts a whole pasted .timer file as well as a bare expression
• Calls out the AND-versus-OR difference between systemd and cron
• Runs entirely locally, with no network requests

How to use

1. Enter an OnCalendar value, or paste a whole .timer file
2. Pick a time zone and how many runs to list
3. Read the upcoming run times and the segment-by-segment explanation
4. Copy the generated .timer and .service

FAQ

How is OnCalendar different from cron?

Two key differences. First, when a weekday and a date are both set, cron uses OR while systemd uses AND: cron `0 9 * * 1` is every Monday at 09:00, whereas systemd `Mon *-*-01 09:00` is "a Monday that is also the 1st", which happens only a few times a year. Second, systemd lets you write several `OnCalendar=` lines to trigger one unit (that is its OR), and accepts named weekdays and months, which reads far better.

What does *:0/15 mean?

It means the 0th, 15th, 30th and 45th minute of every hour. Read `0/15` as start at 0, step by 15, run up to the maximum of the field (59 for minutes), which yields 0, 15, 30 and 45. Reading `0/15` as "0 to 0" leaves only 0 — no error, but entirely wrong.

Why does the time zone change the result?

Because `OnCalendar=` is evaluated in local time. The same `*-*-* 03:00:00` is 03:00 UTC on a UTC machine and 19:00 UTC (the previous day) on a Shanghai machine. So this tool makes the zone explicit and shows both the local time and UTC — check a machine's zone before moving a unit file onto it.

What does `~` mean?

It means a day counted back from the end of the month. `*-*-~1` is the last day of each month and `*-02-~03` is the third-last day of February — and it tracks the month length automatically (in a leap year February has 29 days, so the third-last is the 27th). That is far more reliable than hard-coding 28.

Why does a .timer need a .service?

Because in systemd the timer only decides when to fire; what to run is defined in the service. They usually share a name (`backup.timer` fires `backup.service`), and `Unit=` can point at a different one. This tool produces a working skeleton for both — you will want to replace `ExecStart=` with your own command.