That is not possible. You can't have any timers running in system off sleep. Only pin interrupts can wake the nRF from system off (or NFC). So system on is your only choice. If you don't want the periodic wakeups from the calendar module you need to disable it, or alternatively change it to less frequent interrupts (since this is what is used to keep track of time, you might not want to disable it).

For system off, please check the documentation.

"System OFF is the deepest power saving mode the system can enter. In this mode, the system’s core functionality is powered down and all ongoing tasks are terminated."

meaning no clocks will run.

BR,

Edvin