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UTC is not a timezone. GMT is a timezone that corresponds to the mean solar time as measured by the royal observatory in Greenwich, England. UTC on the other hand, is a time standard for how clocks are synchronized. It's always the same time in UTC and GMT though, so in that respect they are the same.

If you ever get into a situation where there's a problem because you used the wrong one you'll know more than anyone on this sub already!

"Greenwich Mean Time (GMT) is often interchanged or confused with Coordinated Universal Time (UTC). But GMT is a time zone and UTC is a time standard."

There are different time standards. Some add and remove leap seconds based on the real orbit of the Earth. Others don't. All time zones follow UTC with offsets of down to 15 minutes. GMT happens to be UTC+0h00m.

I see! Thanks a lot, I've just been really confused when I've seen them used in different places.

Ok, so that last part just means that GMT and UTC are basically the same time, just measured differently?

There can be as much as a second difference between GMT and UTC times, which is why we’ve been adding leap seconds ever few years to UTC. Starting in 2035, it’s been determined that we will stop adding leap seconds to UTC, so at that point UTC and GMT times will start to drift more, but still likely only by a few seconds. I don’t believe it’s been determined what we’ll do when the gap becomes greater.

The definition is different, in practice, they are the same thing.

Maybe nice to know, UTC is used in aviation worldwide, the aircraft and air traffic control systems as well as schedules, weather reports, etc. all work with UTC, so there cannot be any confusion if flights cross time zones. You never notice because on the ticket you obviously have local times, but behind the scenes it's all UTC, everywhere.

> Starting in 2035, it’s been determined that we will stop adding leap seconds to UTC

I did not know that. I am glad that they are going away. Leap seconds are annoying to deal with.

GMT is now ambiguous and there's nothing official that says what it means. In science, UTC has taken over as the main time standard, based on atomic clocks. The old definition of time used by GMT has been superseded and it's no longer clear what it might mean, though any of several standards based on the rotation of the earth, like UT1 and UT2, could be used.

The name GMT is still used by the British public to refer to the civil time used when summer time is not in effect. But there's no law or regulation that gives a definition of GMT or even how civil time is defined in the UK. The British body responsible for civil time has taken the obvious course and uses UTC.

It’s a very recent development. The vote or whatever was a couple weeks ago.

In 1990, the Royal Observatory in Greenwich stopped observing Greenwich Mean Time due to budget cuts. The responsibility for timekeeping was given to the BBC, which then used GPS time to estimate UTC. (GPS and UTC are not the same, as GPS does not admit leap seconds. See http://leapsecond.com/java/gpsclock.htm for UTC, GPS, Loran, and TAI.)

'Z' is a time zone, just as 'U' (Pacific Standard Time) and 'T' (Mountain Standard Time) are time zones. Every clock set to Zulu time -- be it a cesium or rubidium standard, a wind-up alarm clock, a pendulum-driven grandfather clock, a wristwatch, or a sundial -- is designated as 'Z'.

UTC is properly designated only as 'UTC'.

Your cell phone gets time from your service provider, and that time is nearly or equal to network time, which itself derives from UTC, which in the US comes from WWVB, the 60-kHz ground wave out of Fort Collins, Colorado. Network Time closely matches UTC except for leap second insertion, when UTC seconds run 57, 58, 59, 60, while NT seconds run 57, 58, 59, 59, before both roll over to midnight, the next month, the next day, and for December insertion the next year.

Again, GMT ended 32 years ago. Since then, 'GMT' signifies that the person using it is ill-informed or just lazy. GMT was in no way a time zone.

In the movie "Hidden Figures" they have a launch countdown in UTC, but NASA used GMT at least through Apollo 11 (and perhaps beyond).

The gap may never become wider. Last year it was determined that a leap second should be deducted from UTC for the first time, but there was no precedent on how to do so. This was the driving factor for the removal of Leap Seconds overall.

I'm waiting for reddit to chime in with all the "they are like..." similes.

They are like Sticky Notes and Post-It (tm) notes.

It's like saying, little Timmy is 1 meter tall. Does that mean that Timmy's mom is 1.6 Timmies tall? Technically she is, but it's better practice to measure Timmy's family members in meters instead of Timmies, and say his mom is 1.6 meters tall.

GMT just happens to be UTC+0. They are "the same time", but one of them is the actual measurement tool (UTC), the other is the measurement value as measured by that tool (GMT); in the same way as a meter stick is the tool, and the 1-meter tall Timmy is the measured value.