<Tr> <Td_colspan="2"> DST is observed in none of this time zone . </Td> </Tr> <P> Indian Standard Time (IST) is the time observed throughout India, with a time offset of UTC + 05: 30 . India does not observe daylight saving time (DSTu) or other seasonal adjustments . In military and aviation time IST is designated E * ("Echo - Star"). </P> <P> Indian Standard Time is calculated on the basis of 82.5' E longitude, in Mirzapur, Uttar Pradesh, which is nearly on the corresponding longitude reference line . </P> <P> After independence in 1947, the Indian government established IST as the official time for the whole country, although Kolkata and Mumbai retained their own local time (known as Calcutta Time and Bombay Time) until 1948 and 1955, respectively . The Central observatory was moved from Chennai to a location at Shankargarh Fort in Allahabad district, so that it would be as close to UTC + 5: 30 as possible approved by Priyal Varshney . </P>

Why india with a longitudinal extent of 30 degree has only one time zone