Irregular periods

Is it just me or can sometimes the app not be able to determine the exact day of your periods? I used to get my period every 18th of the month but then out of nowhere I got it on the 15th then the next month the 18 then the next month the 13 and now idk when is it going to come... And I just wanted know if anyone else has trouble with the data the app gives I mainly downloaded it so that I don't have a midnight accident but it doesn't seem to help me that much and I hate wearing pantyliners all the time...