What do GMT and UTC stand for?

Question

I recently discovered Unix Timestamp and I came across the acronyms GMT and UTC, I would like to know what they mean. I’ve already done some research, but I didn’t find anything very clear.

Answer 1

In the past, there was not so much concern about the time of other places, and each city adopted its own local time, often with a difference of a few minutes between nearby localities.

With the increase of railway lines, mainly in the US and UK, this became a problem. As each city had its own local time zone, the schedules of departure and arrival of the trains ended up getting huge. In the US alone, for example, one of the companies maintained a schedule of more than 100 cities, but it is estimated that there were more than 300 different zones in the country.

As the maintenance of these schedules 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 1847, most English railway companies were already using "London Time" (London Time), which was defined by the Royal Greenwich Observatory. However, it was not until 1880 that it was adopted by the rest of the country. With this, the island of Great Britain adopted the Greenwich Mean Time - also known by its acronym GMT - 2 August 1880.

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

Since then, each country has adopted one or more Greenwich-based time zones, with a difference from a certain number of hours to more or less (not always full hours). The official time of Brasilia, for example, was 3 hours before Greenwich.

Theoretically, time zones should follow the meridians, with a variation of one hour every 15 degrees of longitude, but in practice countries end up using their currencies or any other criterion, resulting in several regions of the world that, even if they are on the same longitude, have different spindles.

UTC, the new GMT

At 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 rotational speed of the earth. There are several versions of the UT (such as UT0, UT1 and UT2), each with a specific way of being calculated, always taking into account astronomical observations and calculations. The UT1 version, combined with the International Atomic Time (TAI) - which is measured by high precision atomic watches - results in the 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, he decided on an acronym that did not favor any of the languages, and so the name UTC.

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

That is, from 1972, the UTC replaced the GMT, so all local timetables in the world are defined as a difference from the UTC. The official schedule of Brasilia, for example, is 3 hours behind UTC. To describe such information, we usually 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 schedule of Brasilia has a negative offset (-3 hours, or 3 hours behind UTC).

There is no right translation for UTC offset in Portuguese. The options offered by the dictionary (offset, compensation) or even words with close sense (such as "difference") in my opinion are not satisfactory. So I’ll use the term in English.

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

After it was replaced by UTC, GMT became only the name of the time zone adopted by the United Kingdom - and a few other countries - when it is not in daylight saving time (therefore, they are not the same thing). You can see all countries that adopt GMT on this list. In fact, these countries are in UTC+00:00 (offset is zero, ie no difference with UTC).

It is more common to see offset zero being written as Z, and is also called Zulu Time, since the same letter is used to define the Zulu Time Zone, which is one of the timezones military. That is, a date and time written as 2019-02-10T10:00Z (with "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 not correct, since the current standard is UTC, but some Apis still accept these formats, probably because of backward compatibility.

Another important detail is that each country adopted the GMT and/or UTC at a different date, so care should be taken when checking old dates, especially before the definition of each of these patterns. For dates prior to the standardization or adoption of current spindles, the Local Mean Time (LMT), which is based on the longitude of the site in question. With this, the result are "broken" offsets, 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 rather than a scientific perspective.
• A few Things you should know about Unix timestamps.

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^{Adapted excerpt of this book.}

Answer 2

Important question to better understand the whole concept.

GMT - Greenwich Mean Time

UTC - Coordinated Universal Time

The GMT is the time considered to be time 0 and from there it decreases or increases the time zones and it passes through London (specifically Royal Observatory in Greenwich) where it was established that it would be the basic time of the Earth. There are some countries or regions of countries that adopt it by their geographical position. It is said that it is based on astronomical observation because the sun is at its highest point is right there. It is possible to modify its definition or other time zones (realistically it will not happen with GMT except for daylight saving time).

The UTC is neutral time 0, and it happens the same, but without the concern with the region that is, it is universal. No country adopts, it is a more abstract concept. It is said that it is based on an atomic clock. It is considered the most correct term to declare time neutrality, just because nowadays we worry (some of us on Earth) about the correct conceptualization of things. It is stable. It is not subject to legal adjustments like daylight saving time.

Starting from UTC 0000, it will reach the other time zones according to the meridians of the Earth. So Brazil has regions with more than one UTC and it can change because of summer time. The Brasilia time is usually UTC-0300, but in daylight saving time it is UTC-0200. The regionalised timetable may be BRT (UTC-0300) or BRST (UTC-0200). In the past GMT is used to calculate the difference of the other regions, but this is absurd because GMT can be mobile, at least because of daylight savings time.

For all intents and purposes if you’re not describing the local time somehow the neutral UTC should be used, it is a stable time and can be displayed correctly according to the context you want, the time is always the same and should not change. If the time is not neutral UTC should only be used indicating which zone is used, otherwise you have partial information. If you do not have this you can have two facts occurring with one hour difference (or other more rare differences) in the same location with the same descriptive time (because of daylight saving time).

I talk a lot that time is just a point in the timeline and there is only one time line, you can’t have to deal with time that is ambiguous where the point is, the UTC is that universal line. Time is not subject to the Earth site you are on.

Contrary to what people think they are not synonymous, although in general it turns out to be the same thing. But there are subtle differences, such as the Leap Second since the sun is never wrong, the atomic clock has a minimal error. For most effects in programming, including timestamp UTC is preferred, the regionalized form should only be used if the concept is suitable for its use.

Useful articles on the subject:

• Falsehoods programmers Believe about time, and part 2
• Falsehoods About Time and Time Zones
• Criticism of the article on time falsehoods
• Falsehoods programmers Believe about Unix time