Ovulation dates changing!!

Jamilla
Ok so the woman in me always check and recheck everything. I go in an check my cycle dates and ovulation dates now I know specifically I was SUPPOSED to be ovulating on the 27th of last month it was the highest chance of pregnancy it was a Monday I'll never forget cause me and bf did everything we could to make it home together but our schedules are hectic so day wasted. 
I go in and look and it shows my ovulation date starting on the 30th of last month!!! WTH!!!!