For dates, its really only the early ultrasounds they look at. The later ultrasounds aren't accurate as far as dates are concerned, as it is affected by if you have a big/small baby. So they usually won't change a date if it shows the baby is measuring ahead on a later ultrasound.

For example, my baby was measuring 2-3 weeks ahead of dates in the later u/s's I had (20+ weeks), but I went into labour spontaneously on my due date, so his dates were correct. He was just a large weight baby and also above the 97% line in height, which explains why he measured ahead that much in dates in late u/s's. His dad is tall so he was just genetically a bigger baby than average, and the dates and sizes are just that, averages.

If you had an early u/s, that is far more accurate. My EDD was determined on my 6 week ultrasound as I didn't have an LMP date.

I hope that's what you are asking. If it's not, I'm sorry. I can't believe you are 28 weeks now, you've done so well!