Why do we use Universal (UT or GMT) Time?
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.