I need it to run every, say, 14 days. And at a specified hour (say, 4am). Is this possible with systemd?
The easiest way to get approximately every 14 days is to make it twice a month.
That syntax is explained in
*-*-1,15 is the 1st and the 15th of every month of every year.
If you wanted to try for exactly every fourteen days from when the service started:
But there's a catch here: I think you'd have to have the system up the whole time. There is a
Persistent option to have "the time when the service unit was last triggered...stored on disk" but according to the man page "this setting only has an effect on timers configured with OnCalendar".