Hey carrot,

To share my experience, the clock that you purchase is depending on AC current frequency to get the clock timing to be accurate. The transformer is just stepping down the AC voltage and the frequency still remain the same (50Hz). There after few hours you would see that your clock time is slowing down.

If you have experience in electronic, try to pry open the clock and check the IC model and google for the datasheet. From my experience, there are 2 pinouts that allow you to select 50Hz or 60Hz. Change it from 60Hz to 50Hz and it could save your day .