Changes in my ovulation.

Sherry • My husband and I are 40, we are going to try and conceive our first child together. I am having my tubes untied next mon
Why does the app keep changing my ovulation time? This is the second month in a row it has done this. Up till today the app said my peak time was this Saturday now it is saying it was last Sunday. I don't understand and I am confused. Nothing has changed and I have been logging everyday like normal, any suggestions?