How to count DPO?


Can somebody answer this one for me, pls.. So, I had the Peak ( solid smiley ) reading on OPk Clearblue digital advance on December 26. When I logged in to glow that I had peak on the 26th, it shows that my ovulation was the 27th.. Now, should I count the 27th as DPO since my peak was the 26th or I have to start counting Dpo on the 28th per glow ovulation dated 27th?