Following his initiative, in 1884 representants of 27 countries met at the Conference of the Meridian in Washington, and adopted a hour system that is basically the one that is mantained in our days. This system divides the world in 24 zones or time zones, each one of these regions has approximately 15 degrees of length, the meridian zero, that serves like reference for the rest of zones, is located drawing up a line from the north to the south through Greenwich, United Kingdom.
Each zone that advances towards the east from Greenwich supposes one more hour and one hour less if we advanced in direction to the west.
However, there are several exceptions to this rule, because most of the great countries have a number of time zones inferior to which it should be. China constitutes a more marked example, using the same hour in all its territory. The hour that determines the time zone of Greenwich it is denominated as Greenwich Mean Time (GMT).
Lately the GMT denomination is being replaced by UTC (Universal Time Coordinated). In this case, the calculation of the hour its based on the use of atomic clocks, instead of stars observation (as it happened with GMT hour).