Stamp paper date

Yes Valid .... but when clearly mentioned in the agreement.......

Agreement will be and should be executed on current date ........

The date of the agreement or any clause which states the date on which it is supposed to take effect alone will be the date of the commencment of the agreement.
 

If both the party agree to entering into the agreement from a past date, is possible by mentioning that date in the agreement in the stamp paper.

Date of Agreement remains the date of stamp paper, or thereafter.

But mutually agreed terms can be of before date, i.e. the terms commencement can be earlier date........ (but without any disputed terms/causes of earlier date)