Not sure which app is right . . NEED ADVICE ASAP :)

I don't know which date is my real period date . Which means I don't know when to test . I know you CAN have your period twice in a month , because it'll go on into the next month & the blood flow won't probably start until the next month "technically . I've had that happen to me a couple times . GLOW is telling me to test NOW , because my period is due 9/29 . . My period tracker app is telling me my period is 10/1 . . I'm so confused . Advice needed ASAP .