Period start date??

Hi all. Wondering if this happens to anyone else. Have been TTC for months now and I’m realizing I may be calculating my period start date incorrectly. I always put the start date the first day of having to wear a tampon. But maybe that not correct? Right now I put in the glow app that I started my period two days ago when I started to spot but it wasn’t red blood it was more brownish. So I thought I started my period. Well now it’s been two days of that brownish blood but no actual red period blood. Is that still considered being on my period?!!! I feel like a dummy right now for not knowing this.