I have two different <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">period apps</a> one is <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">period tracker</a> and the other is this app but on glow it says my period is suppose to start on monday(yesterday) and on the <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">period tracker</a> it says i start next week monday the 19th. Why are they both different? I dont know if im late or not. So like how do i know which one is right?