Clomid and cycles

Hey ladies. So I’m ttc baby #2 but I had been missing my period. Well, I “started” my period tonight after it being MIA for two months but it’s just light spotting and it started around 5pm. So my question is, is today considered day one or is tomorrow considered day one?

Only asking because my dr gave me an rx for clomid and I’m suppose to take it on day 5 of my cycle but how do I know what day day 5 is without knowing what day one is first?

I plan to message my doctors office in the morning to find out but I would love everyone’s opinion.

Thanks ladies, be blessed and baby dust to all ❤️