Cycle Day 3 Testing Question

Okay, so this maybe a stupid question but I don’t want to mess this up. So, I have to do cycle day 3 blood work prior to starting IUI but I want to make sure I’m counting my cycle days correctly.

Of course I know the first day of your cycle is day one and so on. However, I heard that if your menstrual starts after 5pm then day one is technically not until tomorrow 🤔 I literally just got my cycle around 8:30ish pm and I typically always count right when I see blood day 1 of my cycle. So is this accurate or no?