Wrong date on resume

What was the magnitude of the wrong date error?

Did you turn a 3 month internship into a 1 year Analyst position, or did you just add a month onto your graduation date? 

This is one of those things that you'll never know, and you'll always look back on it and wonder.

Are you able to speak to anyone at the firm somehow?

I'm sure they will tell you if you didn't pass the background check... try not to worry, 2 to 4 months is pretty trivial 

also my question is do i even tell them or just let it ride?

The background check is mostly about your rap sheet and verifying you weren't acquitted of any crimes (which wouldn't be on rap sheet). They also check to make sure you were employed where you say, but they are not looking into dates super specifically. I believe if you don't clear your background check, they must tell you. Either way, you are entitled to a copy of your background check from the agency that conducts it (I think). They also usually reach out if they can't verify something or if something doesn't line up. My buddy had a high school internship at a bank which was purchased. The background check people reached out to him and asked for more information because the business was no longer registered and there were no employees for them to contact (this was for a major commercial real estate brokerage).