Systemd doesn't start a timer unit

I would like to create a timer that launch a command every 2 months (the first of the month) I already do that with a cron, but I would like to do the same with systemd and get rid of cron. I have a problem because when I start my timer, systemd refused to activate it with the following error: systemd: Starting Start my timer. systemd: myservice.timer failed to enter waiting state: Invalid argument systemd: Failed to start Start my timer systemd: Unit myservice.timer entered failed state. I used the two following units: *myservice.timer* [Unit] Description=Start my timer [Timer] OnCalendar=*-01/2-01 00:00:01 Persistent=true WakeSystem=true [Install] WantedBy=timers.target *myservice.service* [Unit] Description=Start my command [Service] Type=oneshot ExecStart=/usr/local/mycommand.sh [Install] WantedBy=multi-user.target I suppose it doesn't work because it miss a parameter or the OnCalendar parameter is not good. I am on Debian Jessie. Thank you.