BUG: Fix indexing on DatetimeBlock
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Addresses one of the problems behind #26864, but probably doesn't address the statefullness problem there.

I'm pretty sure this problem also affects other EA blocks, will check and add tests if so @jreback any idea what’s up with the CI? ```
/home/vsts/work/1/s/doc/source/whatsnew/v0.25.0.rst:786: WARNING: Unknown interpreted text role "method".
/home/vsts/work/1/s/doc/source/whatsnew/v0.25.0.rst:786: WARNING: Unknown interpreted text role "method".
``` method -> meth in the whatnew thanks @jbrockmendel 