If your main question is how reliable a 6-week ultrasound is, it can be pretty helpful. Was it 6 weeks 0 days? If so it might be a little early for the doctor to see everything he or she might benefit from seeing, but it's better than a fifth-week ultrasound. If it was 6 weeks and some number of days like 6 weeks 3 days or 6 weeks 5 days, that is more helpful.
You named three dates, did you have sex with more than one guy? Is there a particular date you are trying to rule out of the three?
Hi, two days ahead of what?