Hi,
I also had a very regular 28 day cycle, predictable to the day for years!
BUT
I couldnt get a + until-
21 days after ovulation/
37 days after 1st day of LMP/
9 days late for my period !!!
They think I randomly ovulated late.
So instead of being 13+6 today, I am 12+5 (still to update ticker!)
I'd worked out my dates, then to be all changed by the scans
From my calculations from your LMP due date is 15th sept
and from DTD 13th Sept????