Explanation:

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 trains 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. It is, therefore, necessary to adopt the local time of some central meridian of  country as  the standard time for the country. In India, the longitude of 82½° E (82° 30' E) is treated as the standard  meridian. The local time at this meridian is taken as the standard time for the whole country. It is known  as the Indian Standard Time (IST)