How many weeks along? Confused!

Just got my positive pregnancy test yesterday and am trying to understand how far along I am.
First day of last period was 9/11/15.  If I'm not mistaken this would make me 4 weeks tomorrow.  
However, the <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a> says I'm 3 weeks 1 day.
The app also thinks I ovulated around 9/29.  
Thoughts?  Thanks!