从http://en.wikipedia.org/wiki/Coordinated_Universal_Time：

The UTC time zone is sometimes denoted by the letter Z—a reference to
the equivalent nautical time zone (GMT),which has been denoted by a Z
since about 1950. The letter also refers to the “zone description” of
zero hours,which has been used since 1920 (see time zone history).
Since the NATO phonetic alphabet and amateur radio word for Z is
“Zulu”,UTC is sometimes known as Zulu time.

技术上，因为航海时区的定义是基于纵向位置，所以Z时间不完全等同于实际的GMT时间“区域”。然而，由于它主要用作参考时间，只要每个人都使用相同的参考，它适用于地球的哪个区域无关紧要。

再次从维基百科，http://en.wikipedia.org/wiki/Nautical_time：

Around 1950,a letter suffix was added to the zone description,assigning Z to the zero zone,and A–M (except J) to the east and N–Y to the west (J may be assigned to local time in non-nautical applications; zones M and Y have the same clock time but differ by 24 hours: a full day). These were to be vocalized using a phonetic alphabet which pronounces the letter Z as Zulu,leading sometimes to the use of the term “Zulu Time”. The Greenwich time zone runs from 7.5°W to 7.5°E longitude,while zone A runs from 7.5°E to 22.5°E longitude,etc.