What day to count as day 1 of AF? TMI

When I start, I'll have horrendous cramping, light spotting. But I don't go full flow until the next day, or that night at the earliest. What do I count as day 1? My first full day? When symptoms first start? What if I go full flow at 9pm, does that still count as day 1? 
My husband and I just started TTC about two months ago, so this is all new to me.