Confusion over dates

Penster
Is anyone else confused over their cycle length. When I started using glow about a year ago I put that my cycle was 28 days and only recently since TTC I have noticed a discrepancy which is that glow has my next period starting on day 28 meaning that my cycle is 27 days according to glow not 28, anyone else had this problem? 
2 months ago cycle was 24 days and last months it was the standard 28 days but I thought I was 1 day late and got excited 😖
Anyhow anyone got AF due on 9th or 10th May and hoping it doesn't come? Problem is it's not just a 2 week wait is it as I would wait a few days after AF is due to test so it's more like 3 grrrr