The OT trivia line forms on the right. Please take a number.

It could still have been a fake even if the false document dates back to 315 (which we can find no trace of and nor could people a millennium ago for whom this was a more pressing matter). Forging a contemporary document is a far more ordinary activity that forging a centuries old one, but it's the same crime, just with more risk of detection because eye-witnesses to the facts will still be alive to testify.