Set default date in ME23N ?

In transaction me23n ..in header details there is a tab i.e, additional data...

under this tab ...there is validity start and vald end fields.

I have to set validity start date same as doc date

and validi end date equal to doc date + 6 months.

for eg.. if doc date is 01.01.2013 than validi star date must be 01.01.2013

and valid end date must be 01.07.2013..

how can i get this...???

please suggest....