Local time is the time of the day on any longitude indicated by the apparent movement of the sun. It is also called sun time or mean solar time. Local time is determined from longitude 0°, which is the Prime Meridian. The noon or midday (12 o'clock) of a place is when the sun is at its highest point in the sky over that place since there are 360° of longitudes, all the longitudes on the globe cannot have noon at the same time. The difference in local time between one longitude and the next is 4 minutes. The local time at Greenwich meridian is called Greenwich Mean Time (G.M.T) or the Universal time. The earth takes 24 hours to complete a rotation. This means, it takes 1 day to turn 360°, 1 hour to make a turn of 15° and 4 minutes to turn 1°.

Since they rotate from West to East, all places in the East will have their time ahead of Greenwich meridian. Places in the West will have their time behind G.M.T. Places 15° East of Greenwich will be 1 hour ahead and those 15° west will be 1 hour behind.