Time zones are conventionally defined parts of the Earth in which the same local time is accepted.

Before the introduction of time zones, each city used to use its local solar time, depending on the geographical longitude. However, it was very inconvenient, especially in terms of train schedules. The modern time-zone system first appeared in North America in the late 19th century.

For greater convenience (so as not to enter the local time for each degree of longitude) surface of the Earth was conventionally divided into 24 time zones. The boundaries of time zones are determined not by meridians, but by administrative units (states, cities, regions).

This is also done for greater convenience. When moving from one time zone to another, the values of minutes and seconds (time) are usually preserved, only in some countries, the local time differs from the world time by 30 or 45 minutes.

The Greenwich Observatory in the suburbs of London is taken as the reference point (zero meridian or belt). At the North and South Poles the meridians meet at the same point, so the time zones are not usually kept there. Time at the poles is usually equated to universal time, although polar stations sometimes keep their own time.