Due date ???

OK I am so confused can someone help me out?? I went to my OB GYN doctor for my 8 week sonogram scan and they said I am due July 22/2016&  now I'm at my doctors appointment at the pregnancy resource center and they are saying that I am deathly pregnant they did a urine test and it became posited within a minute but they are saying that I am due July 21, 2016 and that my OB must have calculated it wrong?? I'm so so confused?? HELP PLZ LOL!