when do I start counting DPO??

So I have been using the cheap opk's got my first positive (25/03)  at 12.30pm tested a few more times inbetween and just done one now (26/03) 12.30pm and still showing positive - I know it can take between 12-36 hours till I do ovulate but I was wondering shall I class 1DPO today as I got my first positive yesterday or wait till it shows negative and class that has 1 DPO?? So confused any help would be great x