BUG: DatetimeIndex - Period shows ununderstandable error
- [x] closes #13078
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13581?src=pr) is **84.31%**

> Merging [#13581](https://codecov.io/gh/pydata/pandas/pull/13581?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13581   diff @@
==========================================
  Files           138        138          
  Lines         51157      51157          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43132      43133     +1   
+ Misses         8025       8024     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [ba82b51...c957541](https://codecov.io/gh/pydata/pandas/compare/ba82b511c76d87421c8900348efebe4577548ec6...c957541ba12ed18a0f415e8d7ff958db77c009ea?src=pr)
 thanks @sinhrks 
