When is my due date?

Jenn
Glow <a href="https://play.google.com/store/apps/details?id=com.glow.android.nurture">nurture</a> says 9/19/16. I looked on other apps and online and it asked when the first day of my LMP was (12/4/15) and they all calculated it being 9/9/16. It's not that much of a difference but I'm just wondering :)