Use RTCx HW peripheral to emulate "tick" and then convert it to human readable time whenever needed. Sure you need to synchronize the clock first and then correct periodically (but that depends on how precise you have LF clock source and how precise you want to be). There are simple examples how to work with RTC in nRF5 SDK examples and HW "driver" libraries but don't expect this exact code, you will need to write the logic yourself.
