• Since the earth makes one complete rotation of 360° in one day or 24 hours, it passes through 15° in one hour or 1° in 4 minutes.
• The earth rotates from west to east, so every 15° we go eastwards, local time is advanced by 1 hour. If we know G.M.T, to find local time, we merely have to add or subtract the difference in the number of hours from the given longitude, as illustrated below.
• A simple memory aid for this will be East-Gain-Add (E.G.A.) and West-Lose-Subtract (W.L. S.).

Source: ForumIAS