Due Date Calculation

Anyone know how glow calculates your due date? I downloaded glow <a href="https://play.google.com/store/apps/details?id=com.glow.android.nurture">nurture</a> after getting a bfp and it picked 1/22/17 as my due date. All of the online calculators are picking 1/25 based on first day of my last period. Glow has ALL of my information so I trust them to a certain level but not sure what they're using to calculate... TIA