When it comes to time zones, if you’re still talking about Greenwich Mean Time there’s something you should know: it’s old news. Okay, that’s not perfectly correct. In the wintertime, as now, the current time in London (and everywhere else on the time zone) is still referred to as GMT. But Coordinated Universal Time, or UTC, is the modern version. You’ll notice UTC sits right at the top of the Flightradar24 home page, for good reason. It is the central point of reference for time in all things aviation.

Many aviators refer to it as Zulu – which we can probably all admit sounds a lot cooler. So what is this Zulu time? Is it always the same time? Why does it exist and how did it come to be? And why is Greenwich the center of the world, at least when it comes to time zones?

As with so many things in aviation, UTC has plenty of history behind it, as well as being a very useful concept for reducing confusion when operating flights across several time zones. So let’s delve into the terms a little. But first, we need to take a look at that acronym.


You may have noticed that the letters UTC are out of order. Coordinated Universal Time should equal CUT. Perhaps that stands for the French then? A clever guess. The UN, which administers this sort of standardization, does still consider French its official language. But UTC en français is “temps universel coordonné”, or TUC. So what’s with the UTC? It turns out it represents a compromise between the two. How very multilateral. It also looks and reads better than CUT or TUC.

Airline sunset takeoff A321

Airline sunset takeoff A321

As airliners head off on long journeys, it’s essential for everyone to be on the same page when it comes to time zones.

What’s so special about Greenwich?

It may not be called Greenwich Mean Time anymore, and in fact even GMT is sometimes changed to General mean time to make it less tied to one city. However, this is where it all originated and UTC and Zulu both refer to the same exact time.

The original of GMT dates back to Britain’s development as a seafaring nation and a great maritime power. They figured out that by carrying a chronometer set to GMT (which was considered to be at 0 degrees longitude), ships would be able to calculate their own longitude based on the time where they were. That’s a fairly complicated science that deserves its own article, but suffice to say it made GMT a central reference point for oceangoing vessels worldwide, and that also meant that GMT as a kind of “standard” time took hold throughout the world as well. The applications for aviation evolved from there.

UTC and Zulu aviation's time zone RAF Airbus A400M

UTC and Zulu aviation's time zone RAF Airbus A400M

The Royal Air Force of Great Britain has its home base on Zulu time, but it’s not always GMT at Brize Norton (where this A400M was snapped.)

However, GMT is not precise enough to be used in modern technical pursuits such as aviation. That’s partly because the earth’s rotation can vary slightly, and GMT doesn’t account for that variation. To solve that problem, a commission at the UN got together and in the 1960s adopted UTC as its more precise successor.

What is UTC exactly?

UTC is essentially based upon the timing of solar noon at 0 degrees longitude (the so-called “prime meridian”), similar to GMT. However, in the case of UTC the time is fine-tuned based upon a much more accurate atomic clock. It’s not perfect, however. As the earth’s rotation slows ever so slightly, leap seconds need to be added here and there. That’s in order to keep the time within close range of what’s known as UT1 – a variant of universal time based upon the true solar time. That solar time is constant, but the earth’s rotation is imperfect, hence the need for the leap seconds. If your curiosity is piqued, head here for more specifics on all that.

There are moves to head to an even better standard that would eliminate the need for adding leap seconds, but no decision has been reached on that as yet.

The UTC clock on an Airbus A320 flight deck

The UTC clock on an Airbus A320 flight deck

UTC time highlighted on the A320 instrument panel

How to sound like an aviator

Zulu and UTC are interchangeable. Zulu comes from the US military, which assigns global time zones with letter codes. The UTC time zone carries the letter Z, which in the phonetic alphabet is expressed as “Zulu.” And saying Zulu will make you sound more like a fighter pilot. Simple as that.

No daylight savings

Even as local time at zero degrees longitude may change because of daylight savings (when clocks “spring ahead” and the time in London becomes BST, or British summer time), UTC does not change. That’s helpful for coordination of things like flights across time zones, though it can cause confusion for those who are operating on civil time.

So for example if someone says they want to meet at 1500 Zulu, in the summertime that would actually be 1600, or 4pm, in London. In the same fashion, that would be 11am in New York during the summer, but in the winter it would be 10am. A quick rule of thumb to remember is that in the wintertime, Zulu is equal to civil time. In the summertime, in locations that observe daylight savings, there is an offset to consider. Clear as day, right?

As expected it’s often only pilots and members of the military who will refer to an event in Zulu time, and usually not when speaking to civilians. Good thing public-facing airline schedules and communications are expressed in civil time instead.

SQ23 flight path and flight information

SQ23 flight path and flight information

SQ23, the world’s longest flight by distance, crosses through over a dozen time zones, but pilots only care about one—UTC.

Of course even that doesn’t mean all time-related confusion is avoided. For instance when airlines schedule flights at a few minutes past midnight, it often confuses passengers, at least at first. Say a flight departs Hong Kong on December 13 at 00:05 (or five minutes past midnight). That means leaving home and going to the airport on the evening of December 12. What percentage of passengers show up 24 hours late instead? I’d be willing to bet there’s at least a few for each departure. Now imagine if that flight was listed as departing at 1600 Zulu on December 12. Would chaos result? Or might it even be better?

Featured image © Yoshiharu Ozaki

Gabriel Leigh grew up on long-haul flights and has been fascinated by airplanes since he can remember. Now based in Sweden, he writes about transport, travel and more for publications like The New York Times, Monocle and Forbes.


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