How to change Expiration date on Blanket Sales Agreement to be less than system date

 How to change Expiration date on Blanket Sales Agreement to be less than system date:

My case is: current date 19-July-2009 , I want to enter Activation date:01-MAR-2009, Expiration date:31-MAR-2009.

When try to save give me this error:

Blanket sales agreement Expiration date must be greater than or equal to activation date and current date.

so how to enter the Activation date and Expiration date less than current date. or how to set current date in previous date.

Thanks