Disney is right, calculators base you ovulating on cd14 with a 28 day cycle.
I have a 28 day cycle but don't ovulate until cd17.
So, I just adjust my date to have me ovulating on cd14.
E.g. if af starts on 1/4, I would actually put 4/4 as my lmp date a then I'd be ovulating on the same day the calculator says.
Another way, if you know when you ovulated, just count 2 werks before that, and that is what you put in. e.g. if you ovulated on the 18th march, as far as a calculator is concerned, your lmp was 4th march.