[midoridensha]

>I have an old wall clock from Japan that runs on 110v/50Hz.

No, it's 100V, not 110. Japanese mains power is 100VAC, 50Hz in the east and 60Hz in the west. Your clock is from the east, made for Tokyo probably. US 110-120V is a bit high for the clock, but it's close enough that it probably doesn't cause any problems.

It sounds like the inverter you bought isn't terribly accurate, unfortunately.

>Been looking for some other way to get 50Hz AC power...

Actually, it's really simple to get highly-accurate 50Hz power to power your clock. All you need to do is move to Europe. Then you can use a 1:2 step-down transformer to convert the 240V/50Hz power there to 120V/50Hz power, and your clock will be accurate.