Pregnancy test results

I have a question, not sure if I’m gonna be able to word it perfectly but…

Curious to know how far into your cycle (past your supposed period date) you can go before seeing a positive test? (Hope that makes sense!)

Reason I’m asking is pure curiosity, I’m suppose to get my period on the 12 of September (always accurate), and I had a fertility reading done recently that stated I successfully conceived this month and a significant date would be September 28th! ( I know some people don’t believe in this stuff but it brings me some comfort and hope during my ttc journey)