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:

I used the two following units:

myservice.timer

myservice.service

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.