My clinic doesn't count your AF if you get it after 12 noon (ish) if that helps - but it's NOT an exact science, it's a rough timeline.
My clinic was the same. I was told to count day 1 as the day I first had full flow BEFORE 12pm. I can certainly understand your stress though. I got my AF a day or two earlier than I expected for my first cycle and didn't have my meds yet either. Until I had those meds in my hand I was so worried that the cycle would be put off for a month.

Best of luck for Monday