In T+10 im assuming you can end the trade before the 10 days is up. Thats just the deadline.
It's a rolling settlement. So the settlement day rolls forward as each trade date rolls forward.
So Trade 9/4/09 T+10 settlement is 27/4/09 (business days)
Trade 14/4/09 T+10 is 28/4/09
Trade 15/4/09 T+10 is 29/4/09
To close an open trade your +days for the closing trade have to reduce each business day by one (until T+1). You have to be a bit careful about using T+1 as in certain circumsatnces it is a bit to quick to settle (I think it takes about 30hrs or something to settle). If this happened you would have an outstanding debit for the purchase for a few days until the sale settled.
If you need to know about the different settlement dates there is a Rolling Settlement Calendar published.