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For a list of time zones by country, see List of time zones by country. Time zones tend to follow the boundaries of countries and their subdivisions because it is convenient for areas in close commercial or other communication to keep the same time.
Even though 98% of Great Britain's public clocks were using GMT by 1855, it was not made Britain's legal time until Improvements in worldwide communication further increased the need for interacting parties to communicate mutually comprehensible time references to one another.
The problem of differing local times could be solved across larger areas by synchronizing clocks worldwide, but in many places that adopted time would then differ markedly from the solar time to which people were accustomed.
Before clocks were first invented, it was common practice to mark the time of day with apparent solar time (also called "true" solar time) – for example, the time on a sundial – which was typically different for every location and dependent on longitude.
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) because of the elliptical shape of the Earth's orbit around the Sun (eccentricity) and the tilt of the Earth's axis (obliquity).
The first of these companies to adopt standard time was the Great Western Railway (GWR) in November 1840. About , time signals were first transmitted by telegraph from the Royal Observatory, Greenwich.
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 while each city in England kept a different local time.
Local solar time became increasingly inconvenient as rail transport and telecommunications improved, because clocks differed between places by amounts corresponding to the differences in their geographical longitudes, which varied by four minutes of time for every degree of longitude.
Many land time zones are skewed toward the west of the corresponding nautical time zones.
This also creates a permanent daylight saving time effect.