DPO Question...help!

Elizabeth • BFP on 4/10/17 for baby #2 ❤️ [Baby #1 = Jan 2016]
I had a positive OPK on 3/18 (in the PM) & 3/19 (in the AM). I had a negative OPK on 3/19 PM. What day do I use to count for DPO
THANK YOU👶✨