India is a large country. Therefore there is a difference in time in different locations in India. This cause many confusions in time of different locations. For avoiding these confusions in a large country like India, a standard time is used which is followed by every part of that country. This standard time in India is known as IST(Indian Standard Time). This IST passes through Allahabad which lies almost in the middle of India. This IST is the time of Allahabad at that time.
The actual time in Arunachal Pradesh is different from actual time in Gujrat (with respect to location of sun) but the time is same when talking about IST.
This IST is not the actual time of Arunachal Pradesh or Gujrat but it is the standard time of India(taken as time of Allahabad).