Significance of date of agreement in the stamp paper

Hi 

I want to understand what is the significance of the date of signing as mentioned in the agreement and date when the agreement is actually signed. 

For example if the stamp paper is dated 30th march 2011 and its mentioned that this  agreement is being entered into on date 31st march 2011 but the agreement is actually signed by both parties on June 30th 2011 .. then is this agreement valid. 

Regards 

Shyam