When I use the calendar app on my iPhone and change to the today view, it often skips back in time to September 19, 2019 for some odd reason. It has occurred through a number of versions of iOS. I’m currently on 14.5 and have an iPhone 12.
iPhone 12
If there is one specific date that you keep getting flipped back to, see if you have one or more repeating events on that date. Try deleting those events and see if that fixes it. I kept getting flipped back to Feb. 28, 2020. I had numerous (over 30) daily repeating events that day, a few of which continued to today. I deleted them all and that fixed it.
Well, that answers the question of whether Apple was able to fix it with iOS 15. ;/
iPhone calendar keeping skipping back to same date
