See originals: http://www.ntp.org/ntpfaq/NTP-s-time.htm http://weather.msfc.nasa.gov/utc.html  

Cooridinated Universal Time, abbreviated as UTC, is an international official time standard. Weather observations around the world (including surface, radar and other observations) are always taken with respect to UTC.

Prior to 1972, the world's time standard was called Greenwich Mean Time (GMT, 格林尼治平均/标准时间). It was used to set the clocks on ships before they left for a long journey. GMT is based on the mean solar time (平太阳日) which varies a lot by itself. Therefore, it is replaced by UTC, based on a quantum resonance (量子共振) of a cesium (铯) atom, being quite more accurate.

Zero (0) hours UTC is midnight in Greenwich England, which lies on the prime meridian (0° longitude). To convert UTC to local time, one would have to add or subtract the local time zone. China: UTC+8; Germany: UTC+2. UTC is based on a 24-hour clock, therefore, afternoon hours such as 4 pm UTC are expressed as 16:00 UTC (sixteen hours, zero minutes).

U - Universal means that the time can be used everywhere in the world, meaning that it is independent from time zones (i.e. it is not local time).

C - Coordinated means the several institutions contribute their estimate of the current time, and UTC is built by conbining these estimates.

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