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 <a href="">glow app</a> 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.