App differences in fertility/ovulation dates

Jessica

Ive been using an app called <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">period tracker</a> for years to track my period, and it currently shows january 19-25 as my fertile window, with an ovulation date of th 24th.

I added the entire past year of my periods to here when i signed up, and it is showing janury 7-13th as my window, 12th as O day.... idk which one to trust and how to sort it out?