Systemd Timer units

I am trying to create some systemd units that are supposed to start scripts at certain intervals. With Cron, I used an expression like 0 3 */7 * * to start a job every 7 days at 3 a.m. That worked great. With OnCalendar, I have no idea how to implement "every 7 days". Or can I use OnUnitActiveSec here? Additional problem: The computer is not always on at the specified time. The job should therefore be repeated as soon as the computer is available again. I have set Persistent=true for this purpose. However, I suspect that OnUnitActiveSec is reset every time I restart the computer. Or does OnUnitActiveSec refer to the time when the unit was activated with systemctl enabled test.timer?