Because we would become confused if dates were given in their local time zones, we decided to keep all dates in Universal Time. That's using a time that starts at a particular location in England, based on the old Greemwich Mean Time. Most of the world has agreed to use the Universal Time, or UT. So all of the SID data is listed as having been taken at UT Time, or the time it would have been at Greenwich when the data was collected. So, 8 AM in California is 16:00 UT.
Why do we use Universal (UT or GMT) Time?
Determine your time from UT time