The world is divided into 24 time zones, each 15° longitude wide, which is the longitudinal distance the sun appears to travel every hour. The Greenwich meridian 0° longitude is the starting point and the mid-meridian of each zone fixes the time for that zone. Since the earth rotates towards the east, time zones to the west of Greenwich are earlier and to the east are later.
Local standard time can be determined for any area by adding one hour for each time zone going eats and subtracting one hour for each time zone going west. The international Date Line i.e ‘at 180° meridian of longitude’. When this line is crossed going west, the date is advanced one day; when crossed going east, the date becomes a day earlier. The time of day remains the same on both sides of the line.
The standard time zone system has been fixed by international agreement, however some countries establish time zones based on political boundaries or adopt the time zone of a neighbouring countries ,e.g China, covers five time zones but all of china is based on the standard time zone of the capital Beijing.