I spent a day fighting with date-fns trying to get some date calculations working and was to the point I was questioning if my entire approach was flawed because there was no reason I should be spending that much time figuring our some simple date calculations. Eventually I decided to try swapping to Luxon. 30 minutes later and it was all working.

I'm still guessing I misunderstood something fundamental about date-fns, but for now I'm advocating for Luxon.