UTC (Coordinated Universal Time/Temps Universel Coordonné) huh? not CUT? It was a weird compromise acronym half way between French and English. It is a refinement of GMT (Greenwich Mean Time). GMT is a historic time standard derived by observing the sun at the prime meridian at the Royal Observatory, in Greenwich, England. GMT has been supplanted by the more stable SI (Systèm Internationale (metric)) UTC standard based on an atomic clock.
People in Britain and Portugal use Western European time which is UTC +0 in winter and UTC +1 in summer.
BIPM (Bureau International des Poids et Measures) 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, W2003, Vista, W2008, W7-32, W7-64, W8-32, W8-64, W2012, W10-32 and W10-64 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 GMT 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.
Where I live in Victoria BC, in the PST (Pacific Standard Time) time zone, in the winter, midnight UTC occurs at 4 PM local time and in DST (Daylight Saving Time) during the summer at 5 PM. Greenwich, England UTC sees the new day 8 or (7 hours during DST ) earlier than we do.
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