TTC Help

I hope I can explain this right. My period should has started on AUG 21 I’ve taken 2 periods and they have both been negative no big deal because I don’t THINK that we had sex close enough to my ovulation day but last month my periods was not on time and again this month and I’m trying to figure out when my ovulation would be next month should I wait till I get my period and measure of that for my ovulation next month? Or what I don’t know what to do. TIA for any help!