BUG: Fix handling of ambiguous or nonexistent of start and end times in date_range
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Closing a `FIXME` note in the test suite.
lgtm. @mroeschke if you want to add some types great, otherwise pls merge. Thanks. Will add typing in a future PR. 