12dPo?? Ovulation??

Hi, this is a question, iv seen a lot of people adding topics with __dpo (obvs says how many days they are past) but what I want to know is when do you count days past, is it from the day you have the "highest" chance according to glow? Or is it from the very First day that you ovulate, or even the last?? Any comments would help me out as this is out first month txt with the help of glow. & I have Pcos too so want to make sure Im counting correctly, glow says today is my first fertile day,(the green days anyone else's? ) thanks!