[fa icon="calendar"] Publié le 23 November 2018 par Guillaume Gremillet


Behind this clickbait catchphrase is a summary of the different time standards that exist nowadays, and a clarification of the differences between GMT, UTC, ...

Let's do the timewarp again

Of course time conflicts aren't something new or specifically related to the internet. For example, the Romans already had a 24-hours based day but it was divided 12 hours of light and 12 of darkness. It implies implies that the duration of an hour varied with the seasons (45 minutes on the winter solstice and 75 minutes on the summer solstice). Time standards try to answer two important questions: how can we count time accurately and how can we find a common basis to really know, "what time is it ?" .

We have found two ways of doing it :

  • To base time on a human-related concern by measuring an amount of time, visible to the naked eye and to take this measure as a reference. For example : measuring the time between two full moon at their Zenith, and dividing this interval into 28 days, one day in hours and so one until we have a second.
  • To find a fundamental unit of time, known to be reliable (I mean that we can measure two events, between which the amount of time is constant), and count those units. This is what quartz watches do.

The goal is to find a unit of time that we can accurately measure and take that as a base unit.

Choosing the right time reference

The measure of day is the main method we’ve been using for a long time to define GMT: the Greenwich Mean Time. One royal observatory in Greenwich measures on a daily basis the duration of the solar day (defined by the amount of time between two consecutive midnights). It is then possible to determine an average day duration, divide it in 24 parts to make an hour, divide this hour in 60 parts, etc...

The issue with GMT

The GMT method is fairly precise because we follow the duration of the Earth’s rotation, so the computed unit time is for the most part synchronized with the solar time. However, this method is not accurate: the Earth’s rotation rate is not constant. This is due to:

  • The tidal drag: the Earth’s rotation slows down because of the moon gravitational attraction
  • The glacial rebound: the ice caps melting cause the continents to spread a bit more around the globe, gathering a bit more around the poles, which is closer to the axis of rotation. This accelerates the Earth’s rotation as the turntable in this video which accelerates when the hammers get closer to the center

The result of the Earth’s rotation slowing down is that a day currently lasts a 0.8ms more than the 86400 expected seconds, and that this difference is increasing.

Solar time

GMT was officially abandoned in 1972 but we continue to have reference clocks based on mean solar time and the Earth’s rotation. Indeed, we now use more precise methods:

  • UT0: this is the Universal Time of an observatory observing stars and moon, relative to the axis of rotation of the earth. Two different observatories around the globe can compute different UT0.
  • UT1: this is calculated from UT0 around the globe and follows the earth rotation rate with respect to distant stars

Just a second

So, the measure of “one second” is much more complicated than simply taking 1/86400 of a (mean) solar day.

This is why, in 1967, the "official" unit of time was defined as follow :

9,192,631,770 times the period of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. (A.N : Numberwang)

This is a physical phenomenon we can accurately measure and that has the benefit of being a multiplicative way of measuring a second. This increases precision : by dividing a big time interval into shorter pieces is more imprecision-prone than counting several small ones.

Based on this definition, atomic clocks were built to measure precisely the time taken by an electron on a caesium atom to transit between two energy levels (insert complicated quantum mechanics explanations here).

Nowadays, TAI (Temps Atomique International - International Atomic Time in French) is the most precise clock that exists on Earth. It is based on 400 atomic clocks across 50 countries, and has a precision level that reaches the 14th decimal.

Once we have the most precise way of counting time, the next problem is to determine a common starting point in order for two atomic clocks to display the same time.

Tick Tock Goes The Clock

TAI and GPS have a difference of 19 seconds despite both being based on atomic clocks. This difference is due to the initialization date of those clocks :

  • TAI was defined in 1967 from an arbitrary clock that had a 10 second offset from solar time
  • GPS was defined in 1980 and the difference between solar time and TAI was then 19 seconds because of variations of the earth rotation rate

This highlights the issue with atomic time : it is consistent, reliable, precise but it is not realistic. It does not follow the earth rotation speed and the offset gets bigger everyday (It is now around 37 seconds, on October 8th 2018). In 1967, we adopted a new time standard, designed to reconcile both the physical consistency of the earth rotation and the atomic precision of TAI.

Split-the-difference : UTC \o/

Cut & Tuc

First of all, UTC splits the difference with its name:

UTC means "Universal Time Coordinated", it is a compromise between the english "Coordinated Universal Time" and "Temps Universel Coordonné". The goal of this compromise was to avoid any confusion and misunderstanding between languages, using an acronym that does not exist in any of those two languages.

What is so special about UTC ?

The elephant in the room with TAI is this growing offset that exists between solar time and the clock. UTC tries to mix fidelity, reliability and realism by having a clock that does follow UT1 time, but with constant TAI seconds.

The aim is to compensate the Earth slowing down by adding seconds to the TAI clock, in order to limit the difference between UT1 and UTC to maximum 0.9 seconds. This difference is call DUT1 and has varied with a serrated pattern during the years. Those seconds are called "Leap seconds" and can only be inserted on June 30th and December 31st.

In practice, when the difference between UT1 and UTC approaches 0.6 second, the IERS (International Earth Rotation and Reference Systems Service) announces the addition of a leap second and its date.

The consequence is that, for example, 2015-30-06T23:59:60 is a valid date. It is not a mistake! 23:59:60 is the leap second between June 30th and July 1st in 2015.

This leap second caused several crashes and bugs: Twitter announced that tweets that were posted "a minute ago" for in fact tweets sent a day ago. Pinterest, Netflix and Amazon crashed with downtimes of 40 minutes: in brief, it was panic.

Fun facts (it's about time !)

There is no December 30th, 2011 in Samoa Islands

The Samoa Islands are located just on the limit between the +12 timezone and the -12 timezone. In order to have more working days with their main commercial partners, they switched from -12:00 to +12:00 to join Australia and New Zealand.

In 1894, they did the exact opposite change for the same reasons, celebrating July 4th twice.

Don’t trust the internet

Just like me, you probably rely on the internet to set up your computer clock, and just like me you’ve probably been misled by Morocco this year. On October 26th, Morocco decided to keep its clock at summer time. For a while, every computer on earth was wrong about the time it was at Casablanca for example, with several repercussions on meetings, flights…

Those time incoherences between computers and actual time in Morocco are a symptom of the fact that time zones aren’t completely a rational decision based on physical laws, but have a human-biased part. It’s especially true concerning the summer time debate.

How many time zones are there on Earth?

Time zones aren't as simple as one could expect. 24 sub-division of the planet wouldn't give you the joy to deal with +5:45 (Nepal) or +14:00 (Kiribati) time zones.

What is the story behind the Kiribati Islands’ special time zone? This archipelago had islands located on both sides of the dateline (meaning there was a difference of 23 hours between neighbouring islands). In order to unify the 33 islands in the same time zone group, on December 31th 1995, the +13:00 and +14:00 zones were created.

This means that today, there is a total of 37 time zones on earth. And if you differentiate summer time / winter time, there is 43 different time zones !

If you want to learn more about time (yes, I love xkcd)

  • https://xkcd.com/1179/
  • https://what-if.xkcd.com/26/

Liked this article? Interested in building an app with us?

Contact a React Native expert in Paris




Want to rate this article?
Submit Rating

Topics: Calendar, Time Standards, UTC, GMT, UTC vs GMT