Zulu Time Is the Only Clock Pilots Trust, and Even London Isn’t on It All Year

Listen to a recorded airport weather broadcast in Florida and you will hear something like “Information Lima, time one seven five three Zulu.” Read a weather report on a pilot’s tablet in Denver and the timestamp ends in a Z.

It is not Florida time or Denver time. It is the same clock a pilot in Tokyo, Nairobi or London is reading at that exact moment.

That clock is Zulu time. Here is what it is, where the odd name comes from, why aviation refuses to run on anything else, and how to turn it into the time on your own wall.

What Is Zulu Time?

Zulu time is Coordinated Universal Time, or UTC: the time at the prime meridian, 0 degrees longitude, which runs through Greenwich in London. Pilots and controllers say “Zulu” because UTC’s time zone letter is Z, and Zulu is the NATO phonetic alphabet word for Z.

The FAA puts it plainly in its pilot manual: it “uses Coordinated Universal Time (UTC) for all operations,” and “the term ‘Zulu’ may be used to denote UTC.” Zulu never springs forward or falls back, and it is written as four digits on a 24-hour clock, so 1753Z is 5:53 p.m. UTC.

Why “Zulu”? The world is lettered A to Z

The name is older than jet travel. According to the National Institute of Standards and Technology (NIST), nautical time zones were being referred to by letters from the 1920s, and the zone at the “zero” meridian got the letter Z.

NATO first distributed the scheme in a 1950 note, and it survives today as the military time zones. Going east from Greenwich, the letters run A to M. Going west, they run N to Y.

The two halves meet at the International Date Line, where zone M and zone Y share the same clock time but sit a full calendar day apart.

One letter is missing from the sequence. J, or Juliett, was originally left out, and it is now used to mean the observer’s own local time. The US Eastern time zone in winter is R, so 6 a.m. in New York in January is written 0600R, or “zero six hundred Romeo.”

The same Z now reaches far beyond aviation. The international date and time standard ISO 8601 marks a UTC timestamp with a trailing Z, as in 2026-09-28T17:53:00Z, and that suffix is sometimes called “Zulu time” too, because it is the same letter.

Why pilots can’t just use local time

An airliner can cross several time zones in a single flight. If every departure, arrival, weather report and clearance were stamped in the local time of wherever it was written, every one of them would need converting before anyone could compare them.

Take New York to London. AeroCorner’s JFK to Heathrow flight time estimate is 7 hours 30 minutes over 3,442 miles (5,540 km). Leave JFK at 6:30 p.m. on a summer evening and you land at 7:00 a.m. the next morning, London time.

On the two local clocks, 12 and a half hours have passed. In Zulu, the flight left at 2230Z and landed at 0600Z, and the arithmetic takes no thought at all. That is the whole point.

Daylight saving time makes local clocks even less reliable, because countries do not switch on the same day. The US moves its clocks on the second Sunday of March and the first Sunday of November. The UK moves on the last Sunday of March and the last Sunday of October.

In 2026, that meant New York sat four hours behind London instead of the usual five from March 8 to March 29, and will again from October 25 to November 1. Arizona and Hawaii skip daylight saving altogether. Zulu ignores all of it.

That is why so much of aviation is stamped in Zulu. A METAR weather report opens with a group like 281153Z, meaning the 28th of the month at 11:53 UTC. The ATIS broadcast at a towered airport gives its observation time in Zulu, and the North Atlantic track system publishes its daily routes against the UTC clock.

Controllers read times as four separate digits. The FAA’s controller handbook gives 0715 UTC, which is 1:15 a.m. Central Standard Time, as “zero seven one five.” When a controller gives a time check, it is rounded to the nearest quarter minute.

How to convert Zulu time to US time

The FAA’s own table works from standard time: add 5 hours to Eastern Standard Time to get UTC, 6 to Central, 7 to Mountain, 8 to Pacific, 9 to Alaska and 10 to Hawaii. During daylight saving time, each gap shrinks by an hour, because Zulu does not move.

Going the other way, subtract. Here is what the clock says across the US when it is midnight Zulu, 0000Z.

US time zone	Standard time	Daylight time	Local time at 0000Z
Eastern	UTC minus 5	UTC minus 4	7 p.m. (8 p.m. in summer)
Central	UTC minus 6	UTC minus 5	6 p.m. (7 p.m. in summer)
Mountain	UTC minus 7	UTC minus 6	5 p.m. (6 p.m. in summer)
Pacific	UTC minus 8	UTC minus 7	4 p.m. (5 p.m. in summer)
Alaska	UTC minus 9	UTC minus 8	3 p.m. (4 p.m. in summer)
Hawaii	UTC minus 10	No daylight time	2 p.m. all year

So the Florida broadcast from the opening, time 1753 Zulu, was 1:53 p.m. local time if it aired in summer, or 12:53 p.m. in winter. The Zulu figure itself never needs that footnote.

Zulu time vs. GMT: close, but not the same thing

People often use Zulu, UTC and Greenwich Mean Time as synonyms. For a pilot’s purposes they are close enough, but they are not the same clock.

GMT began as the mean solar time measured at the Royal Observatory in Greenwich, and from 1884 until 1972 it was the world’s standard for civil time. It followed the Earth’s rotation, which is not perfectly steady. Today GMT is mostly the name of a time zone, the one Britain uses in winter.

UTC is kept by atomic clocks. NIST describes it as a “paper” time scale, calculated after the fact from the weighted average of more than 300 atomic clocks in more than 60 laboratories. National labs such as NIST and the US Naval Observatory run real-time versions that are often within a few billionths of a second of it.

Even the abbreviation is a diplomatic compromise. In English, Coordinated Universal Time would be CUT; in French, temps universel coordonné would be TUC. The International Telecommunication Union chose UTC so that neither language was favored.

The leap second, and why aviation won’t miss it

Because atomic time and the Earth’s rotation drift apart, UTC has been nudged back into line with leap seconds since 1972. There have been 27 of them, all extra seconds, with the most recent inserted on December 31, 2016.

Airlines have felt them. When a leap second was added at midnight UTC on June 30, 2012, it tripped a Linux bug that took the Amadeus Altéa reservation system offline for about an hour. Staff at Qantas and Virgin Australia had to check passengers in by hand, and flights were delayed.

The leap second may now be on its way out. The timeline below shows how the world’s clock got here.

The urgency comes from a problem nobody has faced before. With the Earth spinning faster, a negative leap second, one taken away rather than added, has become possible. Patrizia Tavella, who leads the time department at the International Bureau of Weights and Measures, told Scientific American that waiting until 2035 carried an estimated 30 percent risk of one.

For pilots, none of this changes the clock on the panel. Zulu is whatever UTC says, and letting UTC drift a little further from the sun makes no difference to a flight plan.

So the next time a weather broadcast says “one seven five three Zulu,” you know what is really going on. Somewhere on the far side of the world, another pilot is looking at a clock that reads exactly the same.