Question

why do we select a standard median

Answer 1

Time along the Standard Meridian of India (82°30' E) passing through Mirzapur (in Uttar Pradesh) is taken as the standard time for the whole country. For convenience, there is one standard time followed, Indian Standard Time (IST), which is the time along the Standard Meridian of India at 82°30' E, five-and-a-half hours ahead of Greenwich Mean Time.