DPO-how do you know!?

So I think this is a really silly question, but we all know that every day matters in your TWW! on the day I think I ovulated, is that day 1 or day 0? For example, I think I ovulated June 1. Would tomorrow (June 8) be 8 dpo or only 7 dpo? I'm confusing myself! Thanks in advance! :)