* John Rouillard <issues@roundup-tracker.org> [20220617 01:37]:
> Thomas I think your desired test cases are handled by changing to:
> 
>   adding the number of days in the next month

No, this doesn't work in months with more days than the next, e.g.:
2022-05-01 + 1 month (30 days) = 2022-05-31

-> still the same month instead of the next

> or
>   subtracting the days in the current month?

This would work.

So either changing the UI part instead of the date calculations is
the right thing here or we have to accept that
date + 1 month - 1 month != date
and just properly document it.

And don't forget that +/- 1 year in February needs to be adjusted,
too:
  2024-02-29 + 1 year = 2025-02-28
  2025-02-28 - 1 year = 2024-02-28 (or -29?)