Confusion 😰

When I first found out I was pregnant I had to go to our local health department to get a pregnancy test done there & get it on paper to take to my doctor. They told me I would have my baby on the 19th of January. I go to my doctor to get my first ultrasound & they said I wouldn't have her until the 31st of January. Now I'm 6 months & they're saying she's measuring big but they're not going to change my date. Should I be planning closer to the first of January or the end of January?