One clock
The FAA “uses Coordinated Universal Time (UTC) for all operations,” and the UK’s radio manual says UTC “is to be used at all times.” A flight from New York to London crosses five time zones; if everyone used local time, departure and arrival times, weather reports and flight plans would never line up. UTC doesn’t change for daylight saving either.
Why “Zulu”
The time zone on the Greenwich meridian has the letter Z, and Z in the phonetic alphabet is Zulu. The US National Weather Service puts it simply: Zulu time is, “for practical purposes, the same as Coordinated Universal Time.” NOAA adds that “the Zulu term stems from military usage, while Coordinated Universal Time is the civilian term.” Before 1972 the same clock was called Greenwich Mean Time (GMT).
How to read it
It’s a 24-hour clock with a Z on the end: 1430Z is 14:30 UTC. On the US East Coast that’s 10:30 in summer and 9:30 in winter. Weather reports (METARs) and NOTAMs are all in Zulu.
