software-RTC adapting the AVR-swRTC to SAMD21

To answer your question:
I'm simply not familiar with all the epoc-timing. So a question like yours with no additional explanation of the different words like "epoc" "unix-time" and different zero-time-basing like 1900 or 1970 and where is used which one does not help because your question assumes more knowledge about it than I have
best regards Stefan