[brohee]

That's a very optimistic assumption, the target is 50Hz but if it is below or over for a long period of time (e.g. high load in winter making it hard to sustain the nominal frequency) there are no provision to make it run faster or slower unless the time drifted by more than 30s (that's possibly only valid for Europe).

More at https://wwwhome.ewi.utwente.nl/~ptdeboer/misc/mains.html

[calfuris]

The standards are tighter in the US, with corrections being triggered by 10/3/2s differences and stopping at 6/0.5/0.5s (Eastern/Texas/Western interconnections). Src: https://www.naesb.org/pdf2/weq_bklet_011505_tec_mc.pdf