Being told 2 different things.

So I have 2 different fertility/period tracker apps, this one and another. My glow chart is saying that I am fertile from the 11-16 of this month. My other app is saying I'm fertile from the 7(today)-12 of June. My glow app has never really been off except for last month when I was extremely stressed and my Af was 16days late. I just don't get why the apps are almost a week apart. I'm going to buy ovulation test today but I don't fully understand how to tell if they are positive or not (I have heard the lines have to be a certain darkness for you to be ovulating). So what do you think I should follow? 7-12 or 11-16