Late period

Why is it that when I tell 'glow' that my period is late, it automatically moves my period back... And changes my start date? My average cycle length is 34 days. I took a test ON day 34 (neg). Today is day 37... I told glow I was late for the second time and it has me now starting on day 39. At any point will the app advise me to take a test? I haven't taken another since day 34 because of the cost & disappointment of always getting a negative.