BIPM Bureau International des Poids et Mesures in Sèvres Cedex France maintains the UTC time standard atomic clock.

UTC is the time system used internally in Java for Date, Calendar and System.currentTimeMillis. Windows NT/W2K/XP/W2K3/Vista use UTC internally to track file dates.

The term GMT is sometimes improperly used as a synonym for UTC. They are close, but not quite the same. The term GMT is sometimes improperly as a synonym used for Western European Time, however neither UTC nor GMT has a summer change. Nobody in Britain would conflate them, but North Americans often do. Java uses the term Greenwich Mean Time inconsistently to sometimes include summer time and sometimes not. Java treats UTC and GMT as synonymous. See the leap second entry on how they are mushed together. See the TimeZone listings.