Before clocks were invented, people marked the time of day with apparent solar time. When well-regulated mechanical clocks became widespread in the early 19th century, each city began to use some local mean solar time. Apparent and mean solar time can differ by up to around 15 minutes (as described by the equation of time) due to the non-circular shape of the Earth’s orbit around the sun (Eccentricity) and the tilt of the Earth’s axis (Obliquity). Mean solar time has days of equal length, and the difference between the two averages to zero after a year.
Greenwich Mean Time (GMT) was established in 1675 when the Royal Observatory was built as an aid to mariners to determine longitude at sea, providing a standard reference time when each city in England kept a different local time.
Local solar time became increasingly awkward as railways and telecommunications improved, because clocks differed between places by an amount corresponding to the difference in their geographical longitude, which varied by four minutes for every degree of longitude. The difference between New York and Boston is about two degrees or 8 minutes, the difference between Sydney and Melbourne, Australia, is about 7 degrees or 28 minutes. Bristol is 2°35′ W(est) of Greenwich (East London), so when it is noon in Bristol, it is about 10 minutes past noon in London. The use of time zones smooths out these differences. [via]
You may not have heard of Sir Sandford Fleming so take a look at the man who standardized time.