struggling

below is an extract from a spreadsheet showing columns P to S and I need to show the last date in P1. for e.g
row 2 under P1 should show 29/01/2010.
row 3 under P1 should show 11/09/2009
row 4 under P1 should show 15/12/2009 and so on. in other words all the latest dates should be showing in column P1.

I would be grateful for any help in this.

P Q R S
P1 P2 P3 P4
24/11/2009
26/11/2009 29/01/2010
08/09/2009 09/09/2009
09/09/2009 10/09/2009 11/09/2009
17/11/2009 15/12/2009
09/11/2009 20/11/2009 14/09/2009
20/11/2009 14/09/2009
14/09/2009
10/09/2009 13/01/2010 08/02/2010 09/02/2010
22/02/2010 30/10/2009 03/12/2009 04/12/2009
30/10/2009 03/12/2009 04/12/2009
03/12/2009 04/12/2009
04/12/2009
11/09/2009 12/02/2010
25/11/2009
18/12/2009 31/12/2009 01/01/2010