If you're getting ready to draft an agreement, and you wish to have it take effect before the date it has actually been signed, you can put a clause in that extends whatever services the contract offers to a date back before the signature date. It's an easy process and doesn't take much time, so there's no reason not to backdate a contract if you wish to do so. Be sure that all parties agree to the backdating before it's signed, however, and don't amend a contract to include a backdate, either. That could be suspect later. Instead, if you decide to backdate the contract after it's been drafted, create a new, clean draft that incorporates the backdating.
Ask both parties to agree to the backdating before it takes place.
Explain in the contract that the agreement was reached prior to the signing date of the contract, if that's the reason for the backdating.
Write a clause that states that, although the contract has been signed on this date, the coverage or benefits actually extends back to a prior date.
Have both parties initial by the clause.
Have both parties, and a witness, sign the completed contract.
In certain situations, such as if you're trying to increase your sales commission, backdating contracts can violate the law. Consult a contracts attorney to be sure that your backdating is legal.