The clocks to use would be our quarts digital battery operated clocks and watches that can be read to the nearest sec.
I don't recommend utility powered clocks. They are too easy to get shut off by blowing a fuse or power outage. Also, electrical AC powered clocks are subjected to automatic adjustment for leap seconds. When leap sec are determined the AC frequency is adjusted to keep the clocks accurate. In other words power companies will skip a cycle of AC voltage for every leap sec.
I do not recommend using clocks or watches which are likely to experience a radical change in average temperature from one period to the next. In other words if you ware a wrist watch the same amount and store it in the same house using the same heat or cooling as the past then uniformity exists. If you move to a new climate or stop wearing the watch then that would need to be recorded, just in case it affects the reading.
I don't recommend using clocks that need the battery changed less than once every year.
Offered by Mike.