Ok so hypothetical question....has anyone ever been told the wrong due date by a month dam near?

Ok so the ultrasound is right even if it's off by a week or so? Cause there is this girl that claim that the baby is somebody else's but her due date is December 14 and it has been that for awhile but her and the dude slept together in February and she slept someone else but claims they were a condom. Idk I'm confused lol

if you have sporadic periods, your date can be off if your ovulation was at a different time than your last period indicates. once you get your first ultrasound, they'll give you your date based on baby's size. mine was off by a week... hope my rambling makes sense...