BUG: Resolve passing datetime strings of the form "1-01-01 00:00:00"
- [x] closes #37071 
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

As highlighted in https://github.com/pandas-dev/pandas/issues/37071#issuecomment-707484947, such a form is actually ambiguous from dateutil's point of view so I'll take this issue upstream to where the ambiguity arises (xarray). Happy for anyone else to pick up if they can see a better solution!