Determining cycle days?

Mary • 20 years old, engaged, mommy to a babygirl born on July 22nd, fur momma to my Yorkie-Poo, Zelda :)

Hi I am just wondering how you all determine the first day of your cycle.

Example: I started having medium spotting last night around 11:30 pm. Would I consider that CD1 or would the next day be CD1 since it's so close to midnight? Just need some opinions please :) thanks everyone