Is GLOW accurate? Advice!?

Hi fellow Glow ladies. I've been logging since Jan 22 when I got my nexplanon removed. My husband and I have been trying since. We have one daughter already she is will be 17 months on May 22. It's been 4 months since logging and my cycle seems like it bounced back normal . I always menstruate monthly on the 15th. But Glow said on the 9th of May I would have started. Today is May 13 (Friday the 13th 😂) and if glow is accurate that means I am 4days late. I even went on about every site relating to menstrual trackers and log on and it says he exact same thing. I am 4 days late. I am so anxious to get a hpt but I am afraid I'll be disappointed with results. What mother wouldn't. Advice! Has Glow ever steer you wrong? Is it accurate? Should I wait till the 15th to see if I get my period? Or should I rush to the store and purchase a hpt?