Dating scan?

Cara • Expecting our first baby Jan 1st 2016! Married my best friend after 8 years together 10/10/14.
Around when do doctors usually confirm a due date? I had my appointment at 8 weeks but they said I'm measure a few days ahead but we will stick with my lmp for now. Is that all there is to dating a pregnancy? Will  they at some point give me a more definitive answer?