When to take a test???

I've been late twice on Glow this month. It changed my potential period start date each time. It shows now it should start on the 24th. How long should I wait to take a test? I haven't had a period since July 15th. And since it keeps changing and I've missed it twice this month according to Glow I'm wondering when it's safe to take a test.. Should I wait one more time and see if it starts on the 24th??