How to Calculate DPO

Katie
Hi all. How do you know the exact DPO? 31st ovulation test strip was light, 1st it was mega dark (darker than control), on the 2nd it was much lighter. I read after getting a positive result ovulation will occur 12-36 hours later. How would I know what DPO I am? :) hope that made sense. Thank you very much for helping me.