Earths rotational speed is effected by multiple factors both local to the planet (such as weather) and external (such as solar system gravicational perturbations).

Your questions inherent assumption that only 1 factor influences it and only at 1 constant rate is incorrect.

The speed actually increases and decreases in mathematically chaotic and therefore difficult to predict ways. The IERS attempt to predict changes up to 6 months in advance with the goal of keeping the delta between the highly accurate "atomic time" and civil UTC time below 0.9 iso seconds by adding or removing leap seconds to UTC at the most mathematically and politically appropriate time of year.

Earths rotational speed is effected by multiple factors both local to the planet (such as weather) and external (such as solar system gravitational perturbations).

Your questions inherent assumption that only 1 factor influences it and only at 1 constant rate is incorrect.

The speed actually increases and decreases in mathematically chaotic and therefore difficult to predict ways. The IERS attempt to predict changes up to 6 months in advance with the goal of keeping the delta between the highly accurate "atomic time" and civil UTC time below 0.9 SI seconds by adding or removing leap seconds to UTC at the most mathematically and politically appropriate time of year.