This sounds good for simplicity when AC power is almost always available - it would be interesting to see what the best solution is for each use case. The time

This sounds good for simplicity when AC power is almost always available - it would be interesting to see what the best solution is for each use case. The time synchronization from AC can drift by a second or more over one day, but it is controlled very well over longer time periods. So if you’re switching back and forth between it and quartz, especially at regular intervals error could somehow accumulate. An interesting thing is emergency power supplies don’t always precisely control this, so after a disaster sometimes all AC connected clocks in an area drift by the same number of minutes. NTP or radio synchronization is probably better in many cases since those are absolute rather than relative references. And GPS should normally be the best although there are cases where 1, 2 or all 3 of those might be unavailable.