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 Greenwich Mean Time. Most of the world has agreed to use the Universal Time, or UT. So all of the SID data are listed as having been taken at UT Time, or the time it would have been at Greenwich when the data were collected. For us, 8 AM in California is 16:00 UT. What time is it for you where you are?
Why do we use Universal (UT or GMT) Time?
Determine your time from UT time