BUG: underflow on Timestamp creation
- [x] closes #14415
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14433?src=pr) is 85.26% (diff: 100%)

> Merging [#14433](https://codecov.io/gh/pandas-dev/pandas/pull/14433?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14433   diff @@
==========================================
  Files           140        140          
  Lines         50640      50640          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43175      43176     +1   
+ Misses         7465       7464     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [921ce47...4b7674c](https://codecov.io/gh/pandas-dev/pandas/compare/921ce47c1a3da6b24965890c622aed61423d3bf7...4b7674ca11fc027fbca4f7878bddb1c64f6f2733?src=pr)
 lgtm. Push again though (you might need to `git commmit --all --amend -C amend` to generate a new has. Want to make sure that appveyor is working. I just turned it back on (was set to old pydata/pandas repo)
 lgtm. rebase and merge.
