We measure the hours of the day by the turning of the earth. The time of day at any particular moment depends on where you are. When it is noon in Greenwich, England, it is midnight along the International Date Line. Times changes as you move east or west, so the earth is divided into twenty-four time zones, one for each hour of the day. Some time zone boundaries zigzag so that people living in one region or country can have the same time.
People communicating across two different parts of the globe often use Greenwich Mean Time.
There are four time zones in the continental United States. They are Eastern, Central, Mountain, and Pacific. Alaska is an hour behind Pacific Time and Hawaii is two hours behind Pacific Time. Almost every nation uses this system of measuring time, but China does not have time zones. China extends across five times zones, but the country has a single time zone.