BUG: Avoid AmbiguousTimeError on groupby
closes #14682 


Could you add your example from the issue as a test? Also a release note in the whatsnew file.
 it seems I've broken something.... I'll try to fix.
 ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14683?src=pr) is 85.20% (diff: 100%)
> Merging [#14683](https://codecov.io/gh/pandas-dev/pandas/pull/14683?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **0.08%**

```diff
@@             master     #14683   diff @@
==========================================
  Files           140        143     +3   
  Lines         50706      50790    +84   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43248      43275    +27   
- Misses         7458       7515    +57   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [45543ec...d90afaf](https://codecov.io/gh/pandas-dev/pandas/compare/45543ec22bb01d5c46eff9384491d218cffddd64...d90afaf01a148a5cea15250dbe28617095a84163?src=pr) lgtm.

cc @rockg  Looks fine. thanks @j-santander 

keep em' coming!