Can estimated conception date at 8 weeks be off by a month?

Not a month: Asking the question, "Can something happen?" will almost always give you a YES answer. That's because something indeed CAN happen, even if it only happens once in a million years. That means asking if something CAN happen is not a useful question. The real question is, "How likely is something to happen?" In real life, ultrasound dating at 8 weeks is not going to be a month off.