So I have ALWAYS had a regulated period, since I first started at 12 (I am now 26). 5 day long period, 28 day long cycle, etc. All the normal expected period cycles. I have entered that into Glow Period Editor and for the last few months it has been spot on, and so the period came with the calendar. However this month the Calendar looks weird... I had my last period on November 29th, and with a 28 day cycle, that would make me due December 27th (yesterday), right? But Glow says I shouldn't start until the 31st. I'm confused because I haven't started my period! So are my calculations incorrect and I shouldn't be due for a period for another 3 days? If so, how is that the case?