services.cron = {
    enable=true;
    systemCronJobs=["02 02 * * * myusername poweroff"];
};

# systemctl status cron                                                                                                                    ~
● cron.service - Cron Daemon
   Loaded: loaded (/nix/store/727vkh4lhw5k8f8s06r8y650696rjmsr-unit-cron.service/cron.service; bad; vendor preset: enabled)
   Active: active (running) since Wed 2016-11-23 02:01:18 CET; 1min 19s ago
 Main PID: 15691 (cron)
   CGroup: /system.slice/cron.service
           └─15691 /nix/store/p8zbmblq6warg0npvp56n715fpcnv38j-cron-4.1/bin/cron -n

Nov 23 02:01:18 MBP systemd[1]: Starting Cron Daemon...
Nov 23 02:01:18 MBP systemd[1]: Started Cron Daemon.
Nov 23 02:02:01 MBP /nix/store/p8zbmblq6warg0npvp56n715fpcnv38j-cron-4.1/bin/cron[15691]: (*system*) RELOAD (/etc/crontab)