UTC (1961- ) stands for “Coordinated Universal Time.” In most cases it is the same as the older Greenwich Mean Time (GMT), the time at the Greenwich Observatory in London. It is a bit different, though, because it follows the atomic clocks, not the sun like GMT.
UTC is the baseline for time used all over the world. It is the time that Big Ben shows.
In school we learned that a day has 24 hours and that each hour has 60 minutes and each minute 60 seconds. That makes a day 86,400 seconds long. Simple enough.
Not any more. Over the years clocks have greatly improved. Now we have atomic clocks. They are so good they keep time better than even the sun itself. But the sun still rules our days, so how do we keep our perfect atomic clocks in step with an imperfect sun? The answer is UTC.
If you looked at any of the atomic clocks right now you would see they are running 33 seconds fast. There is nothing wrong with the clocks themselves – it is the sun that is running 33 seconds slow.
Why is that?
Because the earth turns more slowly than it used to. The moon is slowing it down. Each day is a bit longer than the one before. It is not something that anyone really noticed or cared about until they built atomic clocks.
The definition of a second is based on the length of the day in the year 1821 since it is based on observations made from 1750 to 1892. That is when it was 86400 seconds long. But the earth has been slowing down so that now a day is 86,400.002 seconds long, two thousandths of second longer than it used to be.
So now it takes about seven seconds longer for the earth to complete 36,525 days (10 years) than it did in the 1800s.
It is too late now to change the definition of a second. But that means our atomic clocks run too fast – as if it were still 1821!
What to do?
The answer is UTC.
Atomic clocks have been running since the 1950s. No one touches them or sets them back. Everything else is measured by them, so you do not want to change them. The time they keep, the time that is 33 seconds too fast, is known as TAI.
UTC is TAI with a certain number of leap seconds taken out. That number is now 33. A leap second is added at the end of June or December as needed. They are added so that UTC never runs more than a second faster than GMT (now called UT1 by those in the know).
So why is it UTC and not CUT? CUT works for English but not for the French, who call it “Temps Universel Coordonne.” They would shorten it to TUC, not CUT and they have a lot of influence in these matters. Therefore to make everyone happy it was made “UTC,” which favours no one language and therefore it can be the same in all.
– Abagond, 2006.