Why does my expected period date change?

So I was supposed to ovulate around cycle day 14, and then get my period on cycle day 28, according to the app. Today, on cycle day 26, it tells me my period is expected on day 35 instead. All I track is bbt. If this new prediction is true, then my wedding falls on cycle day 32 instead of new cycle day 7. So when can I test? I'd like to test by new prediction cycle day 30, so that I know for the wedding, which will be 16 days since when I think I ovulated. Is that enough time for an accurate result?