First thing I would suspect would be frequency, however if it is an electronic clock [LED based instead of Analog - motor driven by a synchronous motor], then it would of course have a local oscillator [crystal] which would be the base for the real time clock. Now the possibilities of voltage sags would be suspect.
Check the power quality over time.
We have a typical LED alarm clock that runs just fine while being powered normally from the 120 VAC, but when there is a power failure and it runs on the battery backup [9 Volt Alkaline], it gains an average of 5 minutes. In other words, if the power drops out for a full minute at 11:59 AM, and returns at 12:00 PM, the clock will show 12:05 PM when power is brought up again.
Kind of the reverse of your situation, but does demonstrate the accuracy of a local oscillator.
Scott.
