Date format occasionally messed up

Date format occasionally messed up

Please look at a data record in my Trademaster.
Under Shipment View, there is a client PO PO#100031328. The date at forwarder is shown 5/12/08. In fact it should be 12/5/08.
But when I selected correctly in the calendar December 5, after saving the record, the value becomes May 12 again.

Please look into this!

Thanks