There's no easy answer, since when there is an ambiguity, contradiction, or inconsistency in a contract--and that's what a lease is; a contract--and the two parties disagree about the answer (e.g. if they both happen to agree, there really isn't a problem), then if push comes to shove, the courts try to determine the original intent of the parties from when they signed the document. They will look to any other terms within the contract that may shed light on it; they'll also look at correspondence and emails between the parties which may indicate what they wanted to agree to, early drafts of the contract, etc. So you should look at these elements to see if there is any indicia of the correct date; if not, you could bring the contract and all surrounding documents, correspondence, etc. to an attorney to review in detail for you.