It's an honest mistake that shouldn't (I don't believe) affect your application.
Having said that, now that you've noticed, just call your recruiter/admissions person (I'd call instead of email but your choice) and explain the situation. And they'll advise what next. Likely they'll just update it in their systems. Or may ask that you send an email confirming. But again, I can't see how that would affect your acceptance letter. It's a minor date issue.