Opk question

I use the digital ovulation tests to determine when I'll be ovulating. When I get the smiley face that my LH surge has been detected, is that counted as day 1? I'm trying to figure out how many days past ovulation I am if my LH surge was detected 04/13 note: I started doing it at cd 20 same day i get the smiley 🙂 my App said it was on cd 18 ?