title: datetime dtypes in pandas read_csv
body: <p>I'm reading in a csv file with multiple datetime columns.  I'd need to set the data types upon reading in the file, but datetimes appear to be a problem.  For instance:</p>
  
  <pre><code>headers = ['col1', 'col2', 'col3', 'col4']
  dtypes = ['datetime', 'datetime', 'str', 'float']
  pd.read_csv(file, sep='\t', header=None, names=headers, dtype=dtypes)
  </code></pre>
  
  <p>When run gives a error:</p>
  
  <blockquote>
    <p>TypeError: data type "datetime" not understood</p>
  </blockquote>
  
  <p>Converting columns after the fact, via pandas.to_datetime() isn't an option I can't know which columns will be datetime objects.  That information can change and comes from whatever informs my dtypes list.</p>
  
  <p>Alternatively, I've tried to load the csv file with numpy.genfromtxt, set the dtypes in that function, and then convert to a pandas.dataframe but it garbles the data.  Any help is greatly appreciated!</p>