What counts as Period Day 1?

Growing up I’ve always counted my 1st day as the day I start needing a pad / tampon, regardless of time of day. But a friend argues that it has to do with time of day it starts. Her doc told her that the “1st Day” is really supposed to be the “1st Morning”... So if it starts at 10am on Dec. 1st, that’s “Day 1” but if it starts at 2pm on Dec. 1st, then “Day 1” should be marked on the calendar on Dec. 2. Its all very confusing.

How do you count it?