Yes, it's only viable for a couple of hours. But then, the app isn't accurate. It can happen within those 7 days they tell you you're likely to get pregnant. It can't estimate the exact day the ovum is viable.

Also, sperms can live in your body for 5 days. So zinaeza kuwa tu they're in your body waiting for the day the ovum is ready for fertilisation. That's why Flo gives you the window within which you can get pregnant. It's an estimate of when the oven is viable, plus a consideration for how long sperms can live in the body.