When did it change? : UTC replaced GMT as the basis for the main reference time scale or civil time in various regions on January 1, 1972. I was working as a RADAR Systems Technician on contract to the USAF in the mid 1970's and UTC wasn't fully implemented, it wasn't until the late '70's that the use of it really took hold.

Why did it change? : Snagged from Wikipedia The International Telecommunication Union wanted Coordinated Universal Time to have a single abbreviation for all languages. English speakers and French speakers each wanted the initials of their respective language's terms to be used internationally: "CUT" for "coordinated universal time" and "TUC" for "temps universel coordonné". This resulted in the final compromise of using "UTC".

"UTC" also has the benefit that it fits in with the pattern for the abbreviations of variants of Universal Time. UT0, UT1, UT1R, and others exist, so appending "C" for "coordinated" to the base "UT" is very satisfactory for those who are familiar with the other types of UT.

[ message was edited on: 2009/03/09 ]