Expected period date different on Glow and on Ovia...

Erica
Ladies, I need some assistance.. I'm confused as to why my period date is different on here than is on my other <a href="https://play.google.com/store/apps/details?id=com.glow.android">ovulation app</a> Ovia.. I imputed my last period date as January 9th on both with same cycle length information and that's what I got. Which one is actually accurate? Thoughts please..