I took one of those early response tests a couple days before my period was due. It was negative. I took another test the day my period was late and it was positive. I knew the exact dates everything should have been due and exactly when I ovulated, because we were using clomid to conceive. It's different when trying naturally, unless you know when you ovulated from OPKs, so it might take a week or more after your missed period, if your cycle did something funny this month.