Mains-Powered Digital Clocks

[Bert45]

I have four mains-powered digital clocks. There are two in the kitchen: a microwave oven and a 'normal' oven. There are two upstairs: an alarm/clock radio/CD player and the timer for the central heating/hot water. They do not keep the correct time, apart from the kitchen oven. The microwave clock goes fast, and the clock/radio and the central heating timer go slow. Since they all work off the 50 Hz mains, how is this possible? I set the clock radio by using 'RDS search' function and then set the others from that. There are also digital clocks on my computer screen and on my TV screen. The computer clock is about 6 seconds ahead of the TV clock (Why?). At the moment, the microwave clock is about 5 minutes fast, but I can't remember when I set it -- probably several weeks ago. But how long ago it was set should not matter, as it should keep the correct time to within a second or two. The instructions with the central heating timer say "[It] contains a very accurate digital clock that is factory set." But it was two minutes slow this morning.

[Hymie]

Although the clocks may be powered from the mains, they are not using the mains frequency to keep time, but a conventional quartz crystal.

The mains frequency per se is only accurate to around ~2%, however clocks using the mains frequency for time keeping maintain the correct time because the total number of mains voltage cycles over the period of a week are accurately controlled.

[bhg481]

Your TV will be a digital TV. It receives the signal has has to uncompress it to show your programme and this takes a measurable time. If you have more than one TV (or digital radio) you will find they are probably out of sync with each other and all will be slow compared with an FM radio.

[Bert45]

Thanks, both of you. Hard to say which is the best answer (that should be the better answer, of course), as you have each answered different parts of my question. Since posting my question(s) I have found this: National Grid has a licence obligation to control frequency within the limits specified in the 'Electricity Supply Regulations', i.e. ±1% of nominal system frequency (50.00Hz) save in abnormal or exceptional circumstances.' on the National Grid's website. There is a graph showing that the frequency is generally within 0.2%. I've also read that the number of cycles in 24 hours is kept to 50x60x60x24 (not the number of cycles in a week), but I can't find an authoritative source for that.

[bhg481]

Some devices, such as central heating timers, are actually DC devices with a battery charged by the mains. This allows them to retain their settings in case of power loss but does mean that their time-keeping is not regulated by mains frequency. Some devices have a "real" electric clock, governed by mains frequency and others, such as TV recorders, radios receive their time information via the airwaves so, as I said in my earlier post, they will remain accurate but may have a fixed time-shift from the "real" time because of the time taken to process the signal.
PS I admire your pedantic approach to best/better answer.