Period Prediction Accuracy

I just recently switched to glow from another app. I decided to upload my entire year of periods so that I could get better accuracy. However, my old app shows my period starting on January 4th and glow says I was supposed to start on December 30th.  We have been TTC for 3 months, I'm not going to test until after the 4th. Just wondering if anyone else is having this discrepancy in dates.