You dont need to do math... You only need to use the first day of your last full period to know how pregnant you are. Just find an online "due date calculator" and (same with this app) when it asks you for your last monthly period, or LMP, put in May 28. You should be about 6 weeks along or so, or have gotten pregnant around June 10th-ish.
Agreed with above poster!
See I was putting it as my last but missed period. I appreciate the help