Places that are on the same meridian have the same local ( sun) time.since the earth make one complete revolution of 360 degree in 24 hours, it passes through 15 degree in one hour or 1 degree in 4 minutes.
The earth rotatrs from wwsw to east; hence places east of Greenwich see the sun later and lose time.
A suitable memory acronym can be east gain add (E.G.A) and west lose subtracted (W.L.S).So, it is noon in London (near 0 degree ), 15 degree east will be one hour ahead of london. This means here the time will be 1 P.M. Chennai of 80 degree east will be 5 hours 20 minutes ahead.this means here the time will be 5:20 pm.
To avoid confusion about having many local times within one country, a particular meridian is chosen for the whole country whose time is known as standard Time.
Generally the standard meridians are chosen to differ from the greensGre meridian by the multiples of fifteen degree or seven and a half degree, i.e by exact number of hours or half hours. the world is this divided number of time zones.larger countries like Russia, Canada,USA etc. Have greater east west extensions.
