In the US the grid used to guarantee 5184000 per day with only short-term fluctuations (standard WEQ-006, the exact deviation allowed depends on which grid you are talking about). This was used by e.g. cheap clocks in coffee makers, but also by synchronous motors used by telescopes to align the telescope with the stars.
More recently, with our larger power grid, it is more efficient to deviate from 60 Hz. I do not understand the exact reasons, but for some reason the 5184000 cycle/day guarantee turns out to come with some cost when you have a hundred gigawatts of power spread across a thousand miles. There have been two petitions in North America to remove TEC and allow synchronous clocks to accumulate error, I believe this is going forward but not yet implemented.
More recently, with our larger power grid, it is more efficient to deviate from 60 Hz. I do not understand the exact reasons, but for some reason the 5184000 cycle/day guarantee turns out to come with some cost when you have a hundred gigawatts of power spread across a thousand miles. There have been two petitions in North America to remove TEC and allow synchronous clocks to accumulate error, I believe this is going forward but not yet implemented.
See the section "Time Error Correction": https://en.wikipedia.org/wiki/Utility_frequency
See the section "Accuracy": https://en.wikipedia.org/wiki/Electric_clock