title: Cannot populate NumPy datetime64 arrays
body: <p>I'm trying to create a NumPy array that will subsequently be populated by some datetime values. I can't seem to make it work.</p>
  
  <pre><code>import numpy as np
  t = np.empty(3,dtype='datetime64')
  t
  </code></pre>
  
  <p>I get a <code>TypeError: Invalid datetime unit "generic" in metadata</code>.<br>
  Same if I try :</p>
  
  <pre><code>import numpy as np
  t = np.empty(3,dtype='datetime64')
  t[0] = np.datetime64('2014-12-12 20:20:20')
  </code></pre>
  
  <p>I get:</p>
  
  <pre><code>TypeError : Cannot cast numpy timedelta64 scalar from metadata [m] to  according to the rule 'same_kind'
  </code></pre>