I have the following powerpivot table, and I want to create a measure called Opening_Stock, that calculates the last 'event' (or date) prior to 01/02/2012. Anyone provide any ideas how to do this?

Event Part_Number Month Year Date Qty
10598 ADF345 1 2012 15/01/2012 5
10599 ADF345 1 2012 29/01/2012 4
10600 ADF345 2 2012 15/02/2012 6
10601 ADF345 2 2012 29/02/2012 7
10602 EFT689 1 2012 15/01/2012 5
10603 EFT689 2 2012 29/02/2012 3
10604 DBM165 1 2012 15/01/2012 4
10605 DBM165 1 2012 29/01/2012 5
10606 WTP726 1 2012 15/01/2012 7
10607 WTP726 1 2012 29/01/2012 9
10608 WTP726 2 2012 29/02/2012 8

Thanks