The longitude against which Indian Standard time is set passes through Allahabad, India.
British India did not adopt the international standard time zones until 1905 when the meridian passing through Allahabad at 82.5º east (of Greenwich Meridian) longitude was picked as the central meridian for India, corresponding to a single time zone for the country at 5 hours and 30 minutes in advance of GMT.
This went into force on January 1, 1906. However, Calcutta time was officially
maintained as a separate time zone until 1948. Bombay time was maintained but
only informally until about 1955.
After independence and the partition of British India, Pakistan stayed on Indian Standard Time for three years and adopted Pakistan Standard Time at 5 hours in advance of GMT in 1951.
Centre for Astronomy & Astrophysics, Pune
About IUCAA, Research & Publications....
About PRL, Album, Research Divisions....
Department, Indian Institute of Science, Bangalore
History, Research, Programmes, Joint Astronomy Programme....