In days... how far off can a early ultrasound be to detect date of conception?

Three to Seven: Most early ultrasounds are very accurate, to within a week of conception. If you know when you conceived, and it is within a week, your OB will generally use your LMP date rather than the ultrasound due date.

About 4 days: Early transvaginal ultrasound for pregnancy is accurate to about 3-4 days. The time for accuracy is from about 6 weeks to 12 weeks gestation. We count from the first day of the last menses for OB dating. We can see the pregnancy before 6 weeks but usually just the sac and adding the fetal pole makes things a bit more accurate. Best wishes.