I visted roughly a dozen different health websites and got several different answers.
One site said the ultrasound is accurate within:
4-5 days in the first trimester
7-10 days in the second trimester
10-14 days within the third trimester

Another one said ultrasounds can be off by 5-7 days when done early in the pregnancy but they gave no definition of how early.

A third said it can be off by one week in the 1st trimester, 2 weeks in the 2nd trimester, and 3 weeks in the 3rd trimester.