GMT, as the "mean" within its name would indicate, represented the time zone of a hypothetically average day at Greenwich. GMT disregarded the fluctuations in the normal earth-sun interaction. Thus, noon GMT represented the average noon at Greenwich throughout the year.
Over time, time zones became established based on GMT as being x number of hours ahead or behind GMT. Interestingly, the clock began at noon under GMT so noon was represented by zero hours.
UTCAs more sophisticated time pieces became available to scientists, the need for a new international time standard became apparent. Atomic clocks did not need to keep time based on average solar time at a particular location because they were very, very accurate. In addition, it became understood that due to the irregularity of the earth and the sun's movements, the exact time needed to be modified occasionally through the use of leap seconds.
With this precise accuracy of time, UTC was born. UTC, which stands for Coordinated Universal Time in English and Temps universel coordonné in French, was abbreviated UTC as a compromise between CUT and TUC in English and French, respectively.
UTC, while based on zero degrees longitude, which passes through the Greenwich Observatory, is based on atomic time and includes leap seconds as they are added to our clock every so often. UTC was used beginning in the mid-twentieth century but became the official standard of world time on January 1, 1972.
UTC is 24-hour time, which begins at 0:00 at midnight. 12:00 is noon, 13:00 is 1 p.m., 14:00 is 2 p.m. and so on until 23:59, which is 11:59 p.m.
Time zones today are a certain number of hours or hours and minutes behind or ahead of UTC. UTC is also known as Zulu time in the world of aviation. When European Summer Time is not in effect, UTC matches the time zone of the United Kingdom.
Today, it is most appropriate to use and refer to time based on UTC and not on GMT.