sample-timer.nix

    systemd.services.somemachine-rsync = {
      after = [
        "zfs-import-tank.service"
      ];
      path = [ pkgs.rsync ];
      #this notifies another unit in case it fails, i get a text to my phone.
      onFailure = [
        "unit-status@%n"
      ];
      script = ''
        rsync -Hax --delete somemachine.example.com::root/ /tank/backup/somemachine/
      '';
    };
    systemd.timers.somemachine-rsync = {
      timerConfig.OnCalendar = "*-*-* 3:00:00";
      wantedBy = [ "timers.target" ];
    };