df.at_time NotImplementedError {asof]
The asof functionality int the at_time function gives a NotImplemented Error

`df1 = df.at_time(time(15, 0), asof=True)`

asof is very useful. If implemented it would allow us to use the at_time functionality and work with data that might not be complete, or don't have homogeneous time stamps. It would enable us to get the "closest" observation to the desired time for subsetting.

Thanks

Hernan

`NotImplementedError                       Traceback (most recent call last)
<ipython-input-28-3c10b7e963c9> in <module>()
----> 1 df2 = df1.at_time(time(15, 0), asof=True)
C:\Users\Hernan\AppData\Local\Enthought\Canopy\User\lib\site-packages\pandas\core\generic.pyc in at_time(self, time, asof)
   2767         """
   2768         try:
-> 2769             indexer = self.index.indexer_at_time(time, asof=asof)
   2770             return self.take(indexer, convert=False)
   2771         except AttributeError:
C:\Users\Hernan\AppData\Local\Enthought\Canopy\User\lib\site-packages\pandas\tseries\index.pyc in indexer_at_time(self, time, asof)
   1689 
   1690         if asof:
-> 1691             raise NotImplementedError
   1692 
   1693         if isinstance(time, compat.string_types):
NotImplementedError:`
