You'd travel 45 minutes after the time you were supposed to. Take it from the date the train leaves from its source. If you've booked in the train leaving YPR on 6th, you WILL, surely, leave by the train leaving YPR on 6th no matter what date it arrives in Belgaum.
So by that logic (which I hope doesn't look much confusing), you'd leave on August 7th.