Cycle length

​My periods are normally 28-32 days apart.
 I have set the cycle length on glow to 29 days but the period log says my next period is due on the 31st day after the first day of my last period.  Are there issues with the calendar or am I missing something?
​Also, we have been ttc for our 7th month now. The period just gone was 4 days early and very stop start and I am never early.  Any advice would be welcome.
​Thanks in advance.