Glow or my calendar app? Which one is right?

Lupita • married an anime junkie like me. Love to watch 90`s sitcoms, and draw, paint, cosplay. my fav 50"s sitcom is i love lucy. my fav disney princess is jasmine. looking to make newfriends.
My glow app says my period is due the 11th of next month and my calendar app says my next period is due tomorrow the 1st of February. I plugged in all the right info on both apps so why Isnt my expected period on the same day or around the same day? What should I do? No rude comments plz.