Calculating DPO?

How do you guys calculate your DPO? I had a positive ovulation test on the 15th. Do I count the 16th as 1 DPO? Or no because the 16th is the day I likely ovulated so i would count the day after THAT as day 1? Or lastly, do I go 1 step further and assume I ovulated 2 days after the positive test on the 17th so the 18th would be 1 DPO? Hopefully that’s not too confusing and you ladies totally get me! It’s hard to know exactly!! I was bummed when I got a negative test this morning because I thought I was like 9 DPO but depending on how we calculate I would only be like 6/7 DPO so how do you guys count it?