Confused with this fertile window 🤦🏽‍♀️

Hey ladies! So I have a question. According to this app, my period should have started on Friday. It’s Monday night and still nothing. Any other time I would be thinking I’m pregnant, but I had a cycle that came two days late back in October. So I went to the calendar and clicked the “my period is late” option. Once I did that, it changed my period start date from last Friday, to this Wednesday. Cool. However, it also changed my pervious ovulation window. So now, where it used to have November 15th as my most fertile day ( and my husband and I had sex), it has November 14th as being my most fertile day.

I guess my question is, how are we ever REALLY supposed to track our most fertile days if our periods can be irregular sometimes?? Maybe I’m just frustrated.