How accurate are early us? i measured 6w 1d on 8/22. how likely is it that the date of conception occured later then 7/24 (+/- the 7 days)?

Accurate in a range: Ultrasound pregnancy dating gives a date and a range. The date is an estimate. That means the date is not a single point in time. So, when an ultrasound scan gives a date of 7/24 as the estimated conception date, it actually means it is quite likely to have happened a little before 7/24 or a little after 7/24. This all means a person shouldn't think too much about exactness.