The test might not be accurate, although it should be. You are hardly late though. You should give it more than 1 day. Yes, you were right about the most fertile time. The test is 99.9% accurate when taken the day of a missed period. But the only thing to do is keep testing until you get a positive or start your period. Good luck.
The thing about online fertility calculators is that they are not accurate. Not every woman is the same and they usually treat each case the same. Your conception date could've been over a week span because every woman ovulates at different times every cycle. And most women don't have 28 day cycles. For example, mine last about 32 days to 50 days.
Your best bet of getting a positive result IF you are pregnant is to test two weeks from the date you believe you conceived. If you still get a negative with no period, retest in three days. If still nothing, see your doctor! You could just be under stress or something.
if you were due yesterday..i would think a hpt would be fairly accurate by now...however I have heard of some people not getting a positive on an hpt until like 25 dpo!!! Which would make them 5-6 weeks preggo before getting a positive. So, if i were you, i would wait a few days and if you don't get your period, take another hpt. Good luck.