GMT stands for Greenwich Mean Time. It is a time zone on the basis of which, the world’s time is calculated. Now the question arises what is Greenwich, and why it is used for calculating the time. Greenwich is a place that is situated in London in Greenwich England. Since the earth is round, and there needs to be a rotational line, where things begin and end. And, it is GMT, which runs north to south through Greenwich England that is why it is used for calculating the time.
However, before 1972 the time zone was GMT. But afterward it is replaced by Coordinated Universal Time also called Universal Time Coordinate (UTC). This is because the movement of the earth is not always the same, and that’s why time is calculated according to atomic time also called Coordinated Universal Time, as atomic time is used to calculate the thousand- lakhs part of a second so the atomic clocks are a more stable time base. Initially the term Universal Time was introduced in 1928.
“Indeed, even the Greenwich meridian itself is not quite what it used to be – defined by “the centre of the transit instrument at the observatory at Greenwich”. Although that instrument still survives in working order, it is no longer in use and now the meridian of origin of the world’s longitude and time is not strictly defined in material form but from a statistical solution resulting from observation of all time- determination stations which the BIPM takes into account when co-ordinating the world’s time signals. Nevertheless, the line in the old observatory’s courtyard today differs no more than a few metres from that imaginary line which is now the prime meridian of the world.”(- Howse, D. (1997).Greenwich Time and the Longitude. London: Philip Wilson.)
Now, how is the time calculated? The earth’s longitude that crosses Greenwich is the prime meridian. When we move eastward to the prime meridian the time increases, and when we move westward the time decreases. So the time difference between the most eastern part and the most western part is twenty four hours as there are 24 standard meridians of 15o intervals on the earth. So if at prime meridian it is 12 o’clock, after 150 intervals in eastward it will be 1o'clock. In the same way at after 15o intervals in westward it will 11o’clock.
Moreover, earlier GMT has been used in two different conventions for numbering hours. The astronomical convention, which is dating from the work of Ptolemy, was to refer to noon as zero hours. This astronomical convention contrasted with civil convention, as they were referring to midnight as zero hours dating from the Roman Empire. But finally the civil convention was adopted after 1 January 1925 for astronomy, because of a discontinuity of 12 hours, or half a day (ex. Dec 31.5gmt in 1924 almanac becomes Jan 1.0gmt in 1925 almanac). So the terms UT and UTC always refer to midnight as zero hours is accepted to all.
The time zone “GMT” is used by the United Kingdom (BBC world service, The Royal Navy, and the Met Office), Arab countries like (Middle East Broadcasting Centre and OSN). It is also used by Commonwealth Countries, Australia, New Zealand, South Africa, India, Pakistan, Bangladesh, Malaysia and other Eastern Hemisphere countries. Further, GMT was also adopted by the Railway Clearing house of Great Britain in 1847 as well as by all railway companies, and it is from here that the term “railway time” is derived.
Overall, each country has its own time zone, as due to the rotation of earth day and night is not the same at all places. The sun rises in the east, so at that time there will be morning in the eastern part of the world, whereas there will be night in the western part of the world. In India we follow Indian Standard Time (IST), which is also called GMT+5:30, as India is in the eastern part.