Tracking your cycle, when is day 1

So last month, I put September 11 as my period start day because it was when I saw pink spotting (I thought was implantation but it was AF) that lasted a day and a half, the 12th was light but the 13 was full on regular....I’m asking bc if cycle day was September 11, then Glo says I’m two days late, if start day is 13, then cycle start for this Month is tomorrow? I just need to know and I’m impatient bc I had a similar issue in August with me being off tracking and my cycle started a little later...