1 Answer

It is not UTC it is CUT (Coordinated Universal Time) the  "Earth" time related to the sun it is aligned with Atomic Time, which is held constant by monitoring radio emissions  of atoms.

This process allows for a constant, steady base of time, but  occasionally corrections of milliseconds are needed to align the CUT with the  movement of the Earth and sun. These additional seconds, added every few years, have come to be known as leap seconds.

PST (Pacific Standard time) is related to time on a particular place in time around the Earth, know as Greenwich Mean Time, or GMT.

That is determined on the basis of the motion of the "mean," as it relates to the meridian as observed from the Old Greenwich Observatory in England.

The precise measurement of the sun's movement is not measured because of its tendency to be inconsistent at various times during the  year. Therefore, GMT is arrived at based on mathematical calculations of the sun.