[mpoulton]

What else is on the circuit that it's plugged into?

 

[mpoulton]

I bet KB's got it. Most inexpensive digital clocks use the line frequency for timing, very few use an internal oscillator. Somehow you have it set for 50Hz.

 

[mpoulton]

but this has been multiple clocks. Unless the OP recently moved to the states from an area that used 50hz and brought a bunch of clocks with him or he got a really good deal on clocks on a late night tv informercial, I would doubt it was the 50hz/60hz issue.

That would be hilarious. A whole shipment of 50Hz clocks, unloaded on TV to unsuspecting Americans at GREAT prices!

Really though, if it's not the clock it's probably a harmonic problem caused by some nonlinear load on the circuit. The clocks detect zero-crossings of the AC waveform. Each zero crossing advances the clock by 1/120th of a second. If some device on the circuit is introducing glitches in the voltage waveform that look like additional zero crossings, the clock will run faster. This is likely to be something electronic, like an electronic ballast or switching power supply.