drop from datetimeindex requires a timestamp
See this [SO question](http://stackoverflow.com/a/16613835/1240268):

Apparently (for a DataFrame with DatetimeIndex) drop requires a Timestamp object (this is different from ix/loc which can cunningly work out you meant a timestamp, if fed a string). 

```
In [11]: df1
Out[11]:
                     Price1  Price2  Price3
Date_Time
2012-01-01 00:00:00   63.05   41.40   68.14
2012-01-01 01:00:00   68.20   42.44   59.64
2012-01-01 02:00:00   61.68   43.18   49.81

In [12]: df1.drop(pd.Timestamp('2012-01-01 01:00:00'))
Out[12]:
                     Price1  Price2  Price3
Date_Time
2012-01-01 00:00:00   63.05   41.40   68.14
2012-01-01 02:00:00   61.68   43.18   49.81

In [13]: df1.drop('2012-01-01 01:00:00')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-13-334d00239c8b> in <module>()
----> 1 df1.drop('2012-01-01 01:00:00')

/Users/234BroadWalk/pandas/pandas/core/generic.pyc in drop(self, labels, axis, level)
    373                 new_axis = axis.drop(labels, level=level)
    374             else:
--> 375                 new_axis = axis.drop(labels)
    376             dropped = self.reindex(**{axis_name: new_axis})
    377             try:

/Users/234BroadWalk/pandas/pandas/core/index.pyc in drop(self, labels)
   1296         mask = indexer == -1
   1297         if mask.any():
-> 1298             raise ValueError('labels %s not contained in axis' % labels[mask])
   1299         return self.delete(indexer)
   1300

ValueError: labels ['2012-01-01 01:00:00'] not contained in axis
```

Compare to ix/iloc which don't care:

```
In [21]: df1.loc['2012-01-01 01:00:00']
Out[21]:
Price1    68.20
Price2    42.44
Price3    59.64
Name: 2012-01-01 01:00:00, dtype: float64

In [22]: df1.loc[pd.Timestamp('2012-01-01 01:00:00')]
Out[22]:
Price1    68.20
Price2    42.44
Price3    59.64
Name: 2012-01-01 01:00:00, dtype: float64
```

_Perhaps worth investigating where else this could work/not._
