I would really suggest that you try and do some shuffling so you can go on the 15th. March is pretty late, and I wouldnt want to give up a decently on-time interview for one that late in the season. Honestly, unless it was the birth of my first child, I would be heading to that interview next week. I don't know what your reason for not being able to go is, but if it's something silly like a test in a class or work reasons, cut and go! An acceptance to such a great school would be worth the sacrifice to me!