CLN: Rolling helper attributes
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

`is_datetimelike` was just an internal check and can be in-lined 

`_on` can just be calculated once in the init
sure merge on green-ish