This is the time generally adopted by a particular country, which is usually taken from the central meridian of that country.
Standard time is adopted to avert possible confusion in time. Nigeria for instance adopts her standard time on longitude 1 5°E, which is one hour ahead of the Greenwich Mean Time (GMT).
What this means is that if it is 1:pm in London, France, Accra or Mali at longitude O, all other countries in the east will take their time from GMT from London. So if the GMT in London is 3pm, Nigeria time will be 4pm. NOTE: There are 24 time zones in the world.