What do GMT and UTC mean?

In the old days, there was not so much concern about the time of other places, and each city adopted its own local time, often a few minutes apart between nearby locations.

With the rise of rail lines, particularly in the US and UK, this has become a problem. Since each city had its own local time zone, the tables of departure and arrival times of the trains ended up getting huge. Only in the US, for example, one of the companies kept a table with schedules of more than 100 cities, however, it is estimated that there were more than 300 different zones in the country.

As the maintenance of these time tables became impractical, they decided to standardize the time zones as we know them today. Instead of each city having its own local time, an entire region-which could even be an entire country-would follow the same time.

And so came the GMT

By by 1847, most English railway companies already used the "London Time" (London time), which was set by the Royal Greenwich Observatory. However, it was not until 1880 that this was adopted by the rest of the country. With this, the island of Great Britain adopted the Greenwich Mean Time – also known by the acronym GMT – on 2 August 1880.

Four years later, in October 1884, the International Meridian took place Conference , in Washington D.C., USA. At this conference, it was decided on the use of the Greenwich Meridian as the basis from which all time zones would be defined. One of the main factors that influenced the decision is that the Greenwich Observatory produced the most reliable data of the time.

From then on, each country began to adopt one or more Greenwich-based time zones, with a difference of a certain number of hours to plus or minus (nor it's always whole hours). The official time of Brasilia, for example, became 3 hours before Greenwich.

Theoretically, time zones should follow the meridians, with variation of one hour every 15 degrees of longitude, but in practice countries end up using their currencies or any other criteria, resulting in several regions of the world that, even being at the same longitude, have different zones.

UTC, The New GMT

In the same conference that defined GMT as the standard to be adopted-the International Meridian Conference in 1884 - was also defined the Universal Time (UT ), which is basically defined by the average speed of rotation of the Earth. There are several versions of UT (such as UT0, UT1 and UT2), each with a specific way to be calculated, always taking into account observations and astronomical calculations. The UT1 version, combined with the International Atomic time (TAI) – that is measured by high precision atomic clocks-results in standard UTC (Coordinated Universal Time, or "Coordinated Universal Time").

At the time, English and French discussed whether the acronym should be CUT (because in English the name is Coordinated Universal Time) or TUC (in French, Temps Universel Coordonné). In the end, it was decided on an acronym that did not favor any of the languages, and thus the name UTC emerged.

I won't go into the specific details of how UTC is calculated (if you were curious, see the Wikipedia description . What matters here is the fact that in 1972 it became the adopted standard for time zones.

That is, from 1972, UTC replaced GMT, so all local times in the world are defined as a difference from UTC. The official time of Brasilia, for example, is 3 hours behind UTC. To describe such information, it is customary write as UTC-03:00, -03:00, -0300 or simply -03. this difference is called UTC offset, or simply offset. it can then be said that the official time of Brasilia has a negative offset (-3 hours, or 3 hours behind UTC).

There is no right translation for UTC offset in English. The options offered by the dictionary (offset, offset) or even words with close meaning (like "difference") in my opinion are not satisfactory. So I will use the term in English.

The formats -03:00, -0300 and -03 are defined by the ISO 8601 standard, which defines formats to represent dates and times.

After it was replaced by UTC, GMT became just the name of the time zone adopted by the UK – and a few more countries-when it is not in daylight saving time (so they are not the same thing). You can see all the countries that adopt GMT in this list . In fact, these countries are in UTC+00:00 (the offset is zero, i.e. no difference from UTC).

It is more common to see zero offset being written as Z, and it is also called Zulu Time , since the same letter is used to define the Zulu Time Zone, which is one of the military timezones . That is, a date and Time written as 2019-02-10T10:00Z (with the "Z" at the end), indicates that that date and time refer to UTC.

Despite this, you will still see in many places an offset written as GMT-03:00 or GMT-3, or some other variation. Conceptually it is not correct, since the current default is UTC, but some APIs still accept these formats, probably for backward compatibility issues.

Another important detail is that each country has adopted GMT and / or UTC on a different date, so some care should be taken when checking old dates, especially before the definition of each one of these patterns. For dates prior to the standardization or adoption of the current spindles, the Local Mean time (LMT) is used, which is based on the longitude of the place in question. With this, the result is "broken" offsets, such as UTC -3:06:28 (3 hours, 6 minutes and 28 seconds behind UTC).

You can see more information about GMT x UTC in these Articles:

• differences between them , from a more practical than scientific perspective.
• a few things you should know about Unix timestamps.

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