We need indian standard meridian to calculate time.
-it is important to avoid confusion between the states with in the country.
-it passes from middle of the country taking a longitude of 82 degree 30E which passes from allahabad and 51/2 hrs ahead of greenwhich Mean Time.
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We need standard meridian for India because of the following:
to avoid confusion 
it passes centrally through India
it is lated as the prime meridian to avoid lack of 2 hrs between Arunachal Pradesh to Gujarat.
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