ENH: Arithmetic with Timestamp-based intervals
- [X] closes #35908 
- [X] tests added / passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

@arw2019 - Thanks for the review!
I'll try incorporating these.
Also, the point you've raised about different timezones is interesting, I'll have a look at that. This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. closing as stale. if you want to continue, pls ping and can re-open.