> Also the phone should keep track of system time as epoch time, and convert to local.

Yes, but imagine your local time is US Pacific time, but you have a phone intended to be sold in Mexico, so your phone only has Mexico time zones and MX Pacific Time has no DST. During part of the year, you can use automatic time sync, but during the summer, you disable automatic sync and set the clock so that the time displayed matches local time. Your epoch time is now an hour ahead of properly synched devices, but whatevs, your phone shows the right time and that's what counts.