500
How do we know where one time zone starts and another ends? Picture the world as an orange. Time zones are divided by meridians--imaginary lines that run from North Pole to the South Pole. The meridians split the world into sections, like equal slices of an orange. The prime, or zero, meridian runs through Greenwich, England. By international agreement in 1884, that's been the starting point from which all time is calculated. What can you conclude from the words,by international agreement?
What is all the countries of the world use these time zones