Is it possible for doctors to give you an incorrect due date?

So I was just sitting down and doing the math. My ultra sound tech in the beginning told me it looked like I conceived at the end of March of 2017. Which makes sense. I was sick all through out April and May and finally took a test in the middle of May and confirmed that I was pregnant. When I went to the doctors for my first ultra sound she gave me a conception date of around March 26 but then told me my due date would be January 11. I added it in my head and that seems later than it should be so I used a due date calculator with the conception date she gave me to get an estimated due date and it’s showing me the end of December. Has this happened to anyone else? Am I missing something?