Used to be that the accuracy spec was less than 1 second from midnight to midnight, though you could have errors during the day as it slowed down, but you would correct it later to make up the total error. However there have been recent changes to relax this, making the accuracy less as the mess of separate and different energy providors made it harder to keep the accuracy spec.
Clocks powered by mains it is easy enough to differentiate on power on if the input mains is 50Hz or 60Hz, and set an internal divider to compensate for the clock. This makes a clock that works all round the world, though they can eeasily be confused with UPS or generator power suuply as this can vary quite a lot from a nominal frequency, generators by up to 10% easily from no load to full load.