Simplify algebra in Week and SemiMonth offsets
This PR is careful to only touch Week-base and SemiMonth-based offsets, i.e. has no overlap with #18263.  It does, however, simplify the algebra such that the end result looks a  lot like 18263.

- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
