Glow says one thing .. My cycle says...

Brittney
Ok so last Month glow was wrong with my fertile days. ( I think, I know it can change month to month I didn't use the OPK strips) this month glow said I was fertile and I was taking OPK tests which proved glow to be off by a week so when I logged, glow changed my period start date.. I'm pretty regular tho so now idk when I will start.. 30 days like usual or when glow says.. So this also makes me unsure of when to start testing... Any suggestions ladies??