It is not very exact. The total phase error can be hundreds of cycles. In the synchronous grid of Continental Europe, the deviation between network phase time

It is not very exact. The total phase error can be hundreds of cycles. In the synchronous grid of Continental Europe, the deviation between network phase time and UTC (based on International Atomic Time) is calculated at 08:00 each day in a control center in Switzerland. The target frequency is then adjusted by up to ±0.01 Hz (±0.02%) from 50 Hz as needed, to ensure a long-term frequency average of exactly 50 Hz × 60 sec × 60 min × 24 hours = 4,320,000 cycles per day.[21] In North America, whenever the error exceeds 10 seconds for the east, 3 seconds for Texas, or 2 seconds for the west, a correction of ±0.02 Hz (0.033%) is applied. Time error corrections start and end either on the hour or on the half hour. -- https://en.wikipedia.org/wiki/Utility_frequency https://en.wikipedia.org/wiki/Utility_frequency