Pat O’Connor, of Mosgiel, asks :-

Why do our electric clocks seem to gain about 10 - 15 minutes over a 10 day period whereas the clocks on the microwave and the electric range keep perfect time over the same period.

Tim Armstrong, a physicist with the Measurement Standards Laboratory, Callaghan Innovation, responded.

Every clock maintains its time by an oscillator built to keep the time "ticking". In the past the oscillator was a pendulum that swung back and forth and the time on the clock advanced in response to each pendulum swing.

In modern clocks, the oscillator is usually a crystal made of quartz that produces small electrical pulses (around 5 million times a second). The clock counts these pulses and when it gets to five million it advances the clock by one second and then starts counting from zero again.

Such an oscillator is called a frequency reference and is the basis of all time keeping. Your question is about the time on two clocks but the answer to your question is frequency.

No two oscillators will have exactly the same frequency. Over a long period the time on the clocks will slowly differ because of very small differences in the oscillator frequencies. Even the frequencies of atomic clocks differ by small amounts. The two NZ atomic clocks vary by a rate corresponding to approximately a second in about 5000 years.

The time between your microwave and oven will gradually vary because of small frequency differences between them but you would need to up to a year before the time difference reaches a minute.

It is also possible that for both oven and microwave, the oscillator is the 50 Hz frequency of the mains power supply. By using the same frequency source, they will always show the same time. (Note the 50 Hz frequency may vary during the day but over a whole day is controlled to a precise average of 50 cycles per second and hence the time will remain correct.

I am not sure about the electric wall clock. Its time error seems large for a modern clock. Maybe it is using a motor not locked to the mains as the time source in which case it will be much less accurate than a crystal based clock or watch.