How do you check how long your cycles are

Do you check it after your last period or do you include it with your last period I’m 7 days late if I check it after my last period then it would be 33 days but when I include my last period it will be 39 days today please help I’m 7 days late today and just trying to find answers here’s also a picture of it help please thank you and we baby danced all month just didn’t put post it the past 2 months from being sad