I am almost 100% sure that the Greek police will never accept a mistake. If it's in their system, they will insist that there was no mistake on their side. Moreover, the stamp is a stamp, but the "system" is electronic. If that also shows the wrong date, anything you produce as proof will not convince them to change their records. The only "good" thing about this mess is that you can be certain that nobody will accuse you of overstaying.