Within each time zone, every clock must indicate the same time, and between one zone and the following there is a difference of one hour. In the scientific model upon which time zones are based, each time zone covers 15° of longitude; however, zone boundaries have been adapted to international frontiers (or to regional borders of extensive countries) in order to facilitate commercial activities.

In navigation, clocks are frequently synchronized with the local time of Greenwich, called GMT.

Astronomers, use basically the same system, even though they call it UTC (Coordinated Universal Time).

Another situation, in which time zone boundaries do not coincide with political boundaries, it is also recurrent in many nations. When countries are not excessively extensive, the boundaries of the theoretical time zone have been modified in order to adapt them to the frontiers; for this reason actual time zones do not coincide with the theoretical ones.

Time zones are accepted internationally and, on the contrary, they usually suffer some modification in terms of political situations being developed in different places. Besides, within a same time zone some country may adjust the time forward or backward, because of climate problems, political situation or economical matters, under a national or international perspective.

By the end of the First World War, the strong economical crisis in the countries involved prompted the maximum use of the solar light, bringing about the (summer time); in which clocks are adjusted forward one hour. Later the adjustment was of two hours in summer and of one hour the rest of the year. The situation still remains in many nations that saw in this measure a currency saving opportunity for their countries. Chile among them also agreed on the time change.

This is the reason why the solar time does not coincide with the official time.