It is an interesting math problem. The hangup is there are too many whole-number sources of error.
I guess your clock either didn't have a backup battery, or it was dead.
I don't know if it could amount to a whole hour's difference on the display*, but many of those clocks had a backup battery (usually a 9UV) to keep the clock running during a power outage. The battery usually ran a free-running oscillator divided down to approximately 60 Hz to keep the clock going. (Most of them used the power line's 60 Hz as a time base, so a crystal-controlled oscillator wasn't necessary, but there were/are many variations on this theme.)
An old backup battery could have kept the clock running for a while, and then croaked.
Terry, 230RN
* I guess that's why Doggy Daddy asked about a flip-down display, since thumping on or dropping one of these clocks can result in the little tags flipping over anyhow.