Timers are _systemd_ unit files with a suffix of _.timer_. Timers are like other unit 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.
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:
