The time correction DUT1 (sometimes also written DUT)'s the difference between Universal Time (UT1), which's defined by Earth's rotation, and Coordinated Universal Time (UTC), which's defined by a network of precision clocks. » DUT1 = UT1 − UTC UTC's maintained via leap seconds, such that UTC remains within the range −0.9 s < DUT1 < +0.9 s. The reason for this correction's that the rate of rotation of the Earth isn't constant, due to tidal braking and the redistribution of mass with… (More on Dut1)