How to calculate DPO?

What day do you count as your first DPO? The first day after your green fertile window? Or the first day after a positive opk? I know that a positive opk doesn't mean I have ovulated, it means that I am about to ovulate. My positive opk was on Tuesday but my fertile window on glow ended on Wednesday. So is Wednesday 1 DPO or do I start counting with Thursday?

Vote below to see results!