What is the correct way to calculate my cycle? Confused on how many days are in mine.

Hi everyone I am currently trying to get pregnant with my second child at 36 years old. I tried for the past 3 cycles now and thought there was 30 days in mine. Unfortunately I got my period a couple days early this cycle and I recalculated it and it’s saying 29? Can someone please help me with this to understand it better so I can figure out ovulation days and cycle days etc. thanks so much!