Predicted period changes!

So when my period doesn't come when Glow says it should, I delete the period and it pushed my predicted time to the following week. But it also changes my ovulation time and my dpo days and etc. I try to dtd when I ovulate and for the most part glow is spot on when I ovulate. But my periods are just wacky. How can I be sure I actually dtd when I needed to? I'm trying so hard to TTC but I can't seem to get any of it right. Any advice or anything?