The Earth is divided into 24 time zones so that everyone in the world can be on roughly similar schedules (like noon being roughly when the sun is highest in the sky). The idea to divide the Earth into time zones was proposed by the Canadian railway planner and engineer Sir Sandford Fleming (1827 - 1915) in the late 1870s.
Time zones were first used in 1883 by railroads in order to standardize their schedules. World time zones were determined in 1884, at an international conference in Washington, D.C. Each of the 24 world time zones are about 15 degrees wide and differ by one hour.
Early Time Keeping
Until about 100 years ago, each city set its clocks to local time – noon was the time when the Sun was at its highest in the sky, as viewed from that city. Even neighboring cities needed to set their clocks differently to make this happen. For example, when it was 8:00 in New York City, it was 8:12 in Boston (because Boston is about 3 degrees east of New York). Before modern transportation and communication, this difference didn’t really matter.
A Need for Synchronized Schedules
Once railroads were built, this became very awkward. Train schedules needed to be written using common time settings that everybody agreed to, so the U.S. railroad companies adopted the idea of time zones. This was soon extended internationally, with the world being divided into 24 time zones, each one a long strip from North Pole to South Pole, about 15 degrees of longitude wide. All the people in one time zone would set their clock the same way (to the local time in the center of the time zone).
24 Hours in a Day and 24 Time Zones
Since there are 24 hours in a day, dividing the Earth into 24 time zones meant that everybody was using a time setting very close to their local time – there’s at most about a half-hour difference. So 7:00 am was still in the morning, 12 noon was still in the middle of the day, and 7:00 pm was still in the evening.
But this was much more convenient than the older system of using local time. Most neighboring cities use the same time zone settings. Even if two cities are in different time zones, the time settings always differ by a whole number of hours (1 hour, 2 hours, 3 hours, etc.), making it easy to convert from the time in one time zone to the time in the other.
Today, most countries use this time zone system. (In a few places, clocks are set to be 15 minutes or 30 minutes different from the time according to the standardized time zone system.)
The International Date Line
The prime meridian (zero-degrees longitude) passes through Greenwich, England. Halfway around the world in the middle of the Pacific Ocean (180 degrees from Greenwich) is the International Date Line (IDL), where the date changes across the boundary of the time zone. The entire world is on the same date only at the instant when it is noon in Greenwich, England, and midnight at the IDL. At all other times, there are different dates on each side of the IDL.