Employment Dates Messed Up

I got an offer to work at a BB and I realized my resume has somewhat of a mistake. I worked at a company from December-January paid but my resume says Nov-Feb because was doing some work during November and Feb unpaid. My tax returns only have me working from Dec-Jan but if they call my reference they would say I worked from Nov-Feb.. what should I do? Do the 3rd party companies just look at the tax returns or just call the employers?