Effective date or signing date. The first line often includes a date. This would be the date that the contract was entered into or the date that the contract will become commercially effective, unless the contract states otherwise.
Discrepancies. Be careful with dating the agreement: if the agreement has a cover page, it will likely contain a date and contract footers will, in many cases, also contain a date. Furthermore, the signature words of agreement (immediately preceding the signature blocks) will often also contain a date and many signatories will (despite the fact that no space is reserved for it) write a date next to their signature. Make sure that at least the printed dates are all the same; but preferably, to avoid any confusion at later stage, insert an effective date only once.
Any such dating discrepancy is often due to logistics. For example, if the closing date of a transaction slips by a day or two from the scheduled date, the parties might prefer not to change the date in each of the transaction documents. And when one party signs a contract on a given day and then sends the contract by courier to the other party to sign, the date in the introductory clause could be the day that the first party signed or could be any other date.
Date of consent. Sometimes, the described discrepancies between the date of signing and the economic effectiveness of a contract can only be repaired by requesting the return of documents or by bothering the other party with another change. In these cases, it may be worthwhile to consider that the meeting of minds or the acceptance of the last offer was probably even a few days earlier. That means that the (oral) agreement existed already and may help in antedating a document.