Following the harnessing of the regular 'tick' of the caesium atom the measurement of time entered the atomic age and a whole new level of accuracy; more accurate by far than the rotation of the Earth which is measurably (by atomic standards), slowing down.
The most important atomic time is currently International Atomic Time (TAI) which is the primary time standard in the world today. It is the combined input of many atomic clock arrays around the world, each corrected for known environmental and relativistic effects.
UTC is an atomic time that is adjusted at varying points (leap seconds) to accord with the rotational time UT1.