Zulu Time Converter

Zulu is the word for the letter Z in the NATO phonetic alphabet, and Z is the letter for UTC, Coordinated Universal Time. A time written 1800Z or "1800 Zulu" is 18:00 UTC. The National Geospatial-Intelligence Agency's Radio Navigational Aids (Pub. 117) puts it in one line: the military names its time zones with the phonetic alphabet, and Zulu, "Z", is UTC.

Every other letter is an offset from Z, one hour apart. Going east from Greenwich, A is UTC+1, B is UTC+2 and so on to M at UTC+12, with J left out. Going west, N is UTC-1, O is UTC-2, on to Y at UTC-12. M and Y sit either side of the date line, the same clock time a calendar day apart. The table under the converter is a zulu time conversion chart for all 25 letters, with 1800Z in each.

For the United States that puts Eastern Standard Time on R (Romeo, UTC-5), Central on S, Mountain on T, Pacific on U, Alaska on V and Hawaii on W. A letter names an offset rather than a place, so when a zone moves to daylight time it moves one letter east: Eastern Daylight Time is UTC-4, which is Q (Quebec), and Pacific Daylight Time is T. At 1800Z that gives 1:00 PM EST and 2:00 PM EDT, 12:00 PM CST and 1:00 PM CDT, 11:00 AM MST and 12:00 PM MDT, 10:00 AM PST and 11:00 AM PDT, 9:00 AM AKST and 8:00 AM HST.

UTC itself never changes for daylight saving. The trap is on the other side of the conversion. Someone reading 1800Z in Chicago in July should use CDT, UTC-5, and get 1:00 PM CDT; using CST, UTC-6, gives 12:00 PM CST and is an hour out. Pick the abbreviation in force on the date and the arithmetic is exact.

Some zones sit off the whole hour, and the U.S. Naval Observatory's time zone map marks them with a star on the nearest letter: India's UTC+5:30 is E*, Newfoundland's UTC-3:30 is P*, and UTC+13, where New Zealand Daylight Time lands, is M*. Nepal's UTC+5:45 is marked E with a dagger. The converter shows those letters when one of its zones lands on them.

J (Juliett) is the one letter missing from both charts. In military usage it means local time, whatever the local zone is, so a time written with J carries no offset and cannot be converted until the zone is known.

Converting by hand takes two steps. Find the zone's offset from UTC in force on the date, then add it to the Zulu time: 1800Z plus -5 hours is 13:00, 1:00 PM in New York in winter. If the sum passes 24:00 or drops below 00:00, the date moves with it, which is why 1800Z is already the next morning in Tokyo, UTC+9 and letter I.