Type inference code coerces float column into datetime
```
In [10]: ix = [-352.737091, 183.575577]
    ...: df=pd.DataFrame([0,1],index=ix)
    ...: df.to_csv("/tmp/1.csv")
    ...: df2=pd.DataFrame.from_csv("/tmp/1.csv",parse_dates=True)
    ...: print df
    ...: print df2
             0
-352.737091  0
 183.575577  1
                            0
2105-11-21 22:43:41.128654  0
1936-11-21 22:43:41.128654  1
```

Moved from https://github.com/pydata/pandas/issues/3171.
