Why India has selected the standard Meridian
The local time of places which are on different meridians are bound to differ. For example, it will be difficult to prepare a time-table for a train which cross several longitudes. In India, for instance, there will be a difference of about 1 hour and 45 minutes in the local times of Dwarka in Gujarat and Dibrugarh in Assam(about 4000 km apart). It is therefor necessary to adopt the local time of central meridian of a country as the standard time for the country.
The local time at this meridian is taken as the standard time for the whole country. In case of India it is known as the Indian Standard Time (IST). Some countries, however, have a great longitudinal extent and so they have adopted more than one standard times.
The earth, in general, has been divided into twenty-four time zones of one hour each. Each zone thus covers 150of longitude.