title: Datetime intervals with numpy.arange
body: <p>I'd like to create monthly intervals without iterating using np.arange.  As a simple example, I'd like an array containing a np.datetime64 object representing New Years Day for the range 1990-2000.  I'd think something like:</p>
  
  <pre><code>np.arange(np.datetime64("1990-01-01"), np.datetime64("2000-01-01"), np.timedelta64(1, 'M'), dtype='datetime64[D]')
  </code></pre>
  
  <p>but this raises the error</p>
  
  <pre><code>TypeError: Cannot cast NumPy timedelta64 scalar from metadata [M] to [D] according to the rule 'same_kind'
  </code></pre>
  
  <p>If I switch dtype to datetime[M] I get an array of the months, but of course I want the day.</p>