Why do oven and microwave clocks always show the wrong time?

Oven and microwave clocks measure time by counting the cycles of the alternating current from the power grid (50 Hz in Europe), a cheap but imprecise method. Small fluctuations in grid frequency cause these clocks to gain or lose time. For instance, in 2018, a grid imbalance due to tensions between Serbia and Kosovo caused millions of electric clocks to lag by about 6 minutes. Therefore, these appliance clocks are not reliable for accurate timekeeping, though they suffice for cooking timers.