Measuring DC power is not as complicated, as the voltage and current are constant, so there is no frequency and  the time interval for the integration just defines the average interval of the power calculation.
To measure AC systems the periodic time needs to be known. This cannot be done by taking the normal grid frequency (e.g. 50 Hz) as a constant, due to the fact that the grid frequency is ever changing depending on the balance of the energy supply and the electric load at that exact moment.