BUG: Convert non-dates in xls date cells to number
- [x ] closes #10001 
- [x ] tests added / passed
- [x ] passes `git diff upstream/master | flake8 --diff`
- [x ] whatsnew entry

closes GH10001

pls add some tests
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13042?src=pr) is **84.14%**

> Merging [#13042](https://codecov.io/gh/pydata/pandas/pull/13042?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **-<.01%**

``` diff
@@             master     #13042   diff @@
==========================================
  Files           137        137          
  Lines         50214      50220     +6   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42253      42256     +3   
- Misses         7961       7964     +3   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [b42d1dc...5724ea0](https://codecov.io/gh/pydata/pandas/compare/b42d1dc577be0e1e4bcf8db7b6bddf0b03c158f1...5724ea00b5c73e2e737e5c6bd30dded87ca80b8d)
 Prevoiusly failed tests on the lines that now appear as uncovered according to codecov, I'm not getting this.
 if you want to rebase I think I fixed codecov
 Thanks, seems ok now
 can u post a screen shot of what this excel file looks like
 ![image](https://cloud.githubusercontent.com/assets/10603885/14943461/4eb6e75a-0fda-11e6-814a-29a223e7b572.png)

Hashed cell is this 10^n number
 Same thing with codecov
 you need to rebase on master

`git rebase -i orgin/master`

you are not picking up the change I made recently.
 check now pls
 thanks @kordek 
