Time is measured by the movement of the Earth. The Earth is rotated on its axis. For one rotation, it takes 24 hours, it means the Earth is rotated 360° in 24 hrs. It means 15° in a hour or 1° in 4 minutes. Accordingly, the Earth has been divided into 24 time zones of one hour each.
The places which are situated in the east of Greenwich Meridian experience day or Sunrise earlier than the places lying west to the Greenwich Meridian. Thus, time is ahead in the east than the west places at the rate of 4 minutes per degree of longitude.