title: Extracting the hour from a datetime64[ns] variable
body: <p>I have a column that I've converted to datetime using pandas so it's now in the format datetime64.</p>
  
  <pre><code>LOCAL_TIME_ON 
  2014-06-21 15:32:09
  2014-06-07 20:17:13
  </code></pre>
  
  <p>I want to extract the hour to a new column. The only thing I've found that works is below, however, I get a SettingWithCopyWarning. Does anyone have a cleaner way I can do this?</p>
  
  <pre><code>TRIP_INFO_TIME['TIME_HOUR'] = pd.DatetimeIndex(TRIP_INFO_TIME['LOCAL_TIME_ON']).hour.astype(int)
  </code></pre>
  
  <blockquote>
  <pre><code>C:\Program Files (x86)\JetBrains\PyCharm Community Edition 2016.1.4\helpers\pydev\pydevconsole.py:1: SettingWithCopyWarning: 
  A value is trying to be set on a copy of a slice from a DataFrame.
  Try using .loc[row_indexer,col_indexer] = value instead
  
  See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
    '''
  </code></pre>
  </blockquote>