My dates changed by a week and a day - had my NHS scan then a few weeks later a private one and I expected to be 9w4d and I was 10w5d!
With my first baby, the private scan dated me 20th October and the NHS one, 17th October. So not too different.
Dates before 12 week can change a few times
Early private scans can be inaccurate as the baby is literally mm long, so if it's not positioned exactly right or they don't measure exactly end to end, it can put the EDD out by a bit.