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Greenwich Mean Time (GMT) is international time, the basis of the world time clock. It helps eliminate confusion across multiple time zones. It is also known as UTC (Coordinated Universal Time). Aviators commonly refer to it as Zulu time.
Aviators use this worldwide, 24-hour time to eliminate confusion about "a.m." and "p.m." and different time zones. It makes time comparisons and conversions easier.
Converting 12-hour to 24-hour time:
Greenwich, England, was chosen as "zero hour" because it is longitude 0 degrees 0 minutes 0 seconds. If you live in the eastern United States, your local time is GMT minus 5 hours (4 hours for daylight-saving time [DST]); if you live in the West, your local time is GMT minus 8 hours (or 7 for DST). So if you're in New York City and it's noon, the Zulu Time is 1700Z (or 1600Z for DST). Because Zulu Time is based on a 24-hour clock, if it's 5 p.m. (DST) in New York City, that would be 2100Z (5 p.m. is 1700 hours, so it's 17 plus 4).
For more information, see http://wwp.greenwichmeantime.com.
AOPA thanks our members for their continued support in protecting the freedom to fly.