There are a lot of time zones passing through India. now it became very hard to understand what the time is cross india so an indian standard time[IST] was created which made time all over india the same. this is why sun rises quickly in arunachal pradesh and late in gujarat.
From gujarat to arunachal pradesh there is a time lag of two hours. hence the time along the standard meridian of india passing through mirzapur (in uttar pradesh ) is taken as the Standard time of the whole country.the latitudinal extent influences the duration of the day and night , as one moves from south to north.