BUG: ensure consistent behavior of Index.is_all_dates (#19204)
- [x] closes #19204
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
@addammy can you rebase > @addammy can you rebase

Done @addammy can you try to get the CI passing can you merge master; ping on green. closing as stale
if u want to continue pls ping