From NixOS Wiki

Timers are systemd unit files whose name ends in .timer that control .service files or events. Timers can be used as an alternative to cron. Timers have built-in support for calendar-based events and monotonic time events, and can be run asynchronously.


The following example timer runs a systemd unit every 5 minutes which invokes a bash script.

systemd.timers."hello-world" = {
  wantedBy = [ "" ];
    timerConfig = {
      OnBootSec = "5m";
      OnUnitActiveSec = "5m";
      Unit = "hello-world.service";
};"hello-world" = {
  script = ''
    set -eu
    ${pkgs.coreutils}/bin/echo "Hello World"
  serviceConfig = {
    Type = "oneshot";
    User = "root";

The following example starts once a day (at 12:00am). 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.

  timerConfig = {
      OnCalendar = "daily";
      Persistent = true; 

More examples can be found at the Arch Wiki and at the systemd.timer manpage.


List active timers and their current state:

systemctl list-timers

Manually run a service once for testing purposes:

systemctl start hello-world