Greenwich Mean Time (GMT) is so named, because the longitudinal line after which it is named passes through Greenwich, London. Although GMT and Coordinated Universal Time (UTC) share the same time there is a fundamental difference between them. GMT is a time zone. It is used by some African and European countries. UTC on the other hand is a time standard. It serves as the foundation for time zones worldwide. Hence no country officially uses UTC as its local time. They both remain constant and are not affected by daylight saving time (DST) although some countries observing GMT use DST in the summer.
UTC serves as the basis for civil time. This standard is maintained using extremely precise atomic clocks along with calculations of the Earth’s rotation. This standard is maintained, synchronized and coordinated worldwide. UTC is determined by International Atomic Time (TAI) and Universal Time. TAI is a combination of the output of 400 extremely precise atomic clocks that provides the precise calculation of the speed at which our clocks should tick. Universal Time is the time it takes the Earth to complete a turn on its axis and is used to validate TAI against an actual Earth day.