Period always early on Glow

Hey everyone. I have been using glow for 2.5 months now. My period is always 2 days early than Glow predicts. I changed the date manually each time. Does glow eventually begin to get it right on its own after a few trackings or do I need to do something? I'm logging everything now so at the end of summer My husband and I can conceive. I really want to get my ovulation dates and everything as accurate as possible.