 BUG: Decrement start date with a negative, non-Tick frequency
- [x] closes #23270
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @mroeschke! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/indexes/datetimes/test_date_range.py`](https://github.com/mroeschke/pandas/blob/0791095550ffccdb601c3ac22a43293b6f78b0d2/pandas/tests/indexes/datetimes/test_date_range.py) !

 - There are no PEP8 issues in the file [`pandas/tseries/offsets.py`](https://github.com/mroeschke/pandas/blob/0791095550ffccdb601c3ac22a43293b6f78b0d2/pandas/tseries/offsets.py) !

 lgtm. pls rebase after CI is green (soon). thanks!