Timers are_systemd_unit files with a suffix of_.timer_. Timers are like otherunit configuration files and are loaded from the same paths but include a`[Timer]`section which defines when and how the timer activates. Timers are defined as one of two types:
-**Realtime timers**(a.k.a. wallclock timers) activate on a calendar event, the same way that cronjobs do. The option`OnCalendar=`is used to define them.
-**Monotonic timers**activate after a time span relative to a varying starting point. They stop if the computer is temporarily suspended or shut down. There are number of different monotonic timers but all have the form:`On_Type_Sec=`. Common monotonic timers include`OnBootSec`and`OnUnitActiveSec`.
For each_.timer_file, a matching_.service_file exists (e.g.`foo.timer`and`foo.service`). The_.timer_file activates and controls the_.service_file. The_.service_does not require an`[Install]`section as it is the_timer_units that are enabled. If necessary, it is possible to control a differently-named unit using the`Unit=`option in the timer's`[Timer]`section.
List timers:
```shell
systemctl list-timers
```
## Examples
### Monotonic timer
A timer which will start 15 minutes after boot and again every week while the system is running.
`/etc/systemd/system/foo.timer`
```
[Unit]
Description=Run foo weekly and on boot
[Timer]
OnBootSec=15min
OnUnitActiveSec=1w
[Install]
WantedBy=timers.target
```
### Realtime timer
A timer which starts once a week (at 12:00am on Monday). When activated, it triggers the service immediately if it missed the last start time (option`Persistent=true`), for example due to the system being powered off:
`/etc/systemd/system/foo.timer`
```
[Unit]
Description=Run foo weekly
[Timer]
OnCalendar=weekly
Persistent=true
[Install]
WantedBy=timers.target
```
When more specific dates and times are required,`OnCalendar`events uses the following format:
`DayOfWeek Year-Month-Day Hour:Minute:Second`
An asterisk may be used to specify any value and commas may be used to list possible values. Two values separated by`..`indicate a contiguous range.
In the below example the service is run the first four days of each month at 12:00 PM, but_only_if that day is a Monday or a Tuesday.
