confused about DPO need help!!!

So Glow predicts by my opks that confermed ovulation that I have 11 days left till af my question is I want to start testing early but it's so confusing to me by Gows calculation I'm 1 DPO today but it doesn't makes sense to me basically when I'll be 6 days before period witch would put me at 8 DPO by Glow all be only 5 my head is spinning need help guys please?!!!!!