title: numpy Invalid datetime unit generic in metadata error
body: <p>I'm working on updating someone else's code from numpy 1.6.1 to numpy 1.8.</p>
  
  <p>The code that is giving an error(and my added print, which won't even work):</p>
  
  <pre><code>        for name in arr.dtype.names:
              print name
              print arr.dtype.names
              print arr
              print arr[name]
              set_to_nan(arr[name])  
  </code></pre>
  
  <p>The printout and then the error:</p>
  
  <pre><code>time
  ('time', 'nsec', 'SeqNum', 'message_offset', 'event_type', 'match_event_locator', 'trdpx', 'trdqty', 'trdaggr', 'trdtyp', 'trdid', 'bid', 'ask')
  </code></pre>
  
  <p>[ <code>(None, 32128, -1146166105165789278, 1, 0, 0, 0.0, 0, 0, 0, '', [(0.0, 0, 0), (0.0, 0, 0), (0.0, 0, 0), (0.0, 0, 0), (0.0, 0, 0), (0.0, 0, 0), (0.0, 0, 0), (0.0, 0, 0), (0.0, 0, 0), (0.0, 0, 0)], [(0.0, 0, 0), (0.0, 0, 0), (0.0, 0, 0), (0.0, 0, 0), (0.0, -938012672, 5), (0.0, 536870912, 25211), (0.0, 0, 12288), (1.4916681462400413e-154, 527891, 0), (0.0, 48070656, 10), (0.0, 1879048192, 1)])]</code></p>
  
  <p>Error from trying to print arr[name]</p>
  
  <pre><code>  File "/home/venv/lib/python2.7/site-packages/dat/array_utils.py", line 500, in set_to_nan
      print arr[name]
    File "/home/venv/lib/python2.7/site-packages/numpy/core/numeric.py", line 1615, in array_str
      return array2string(a, max_line_width, precision, suppress_small, ' ', "", str)
    File "/home/venv/lib/python2.7/site-packages/numpy/core/arrayprint.py", line 454, in array2string
      separator, prefix, formatter=formatter)
    File "/home/venv/lib/python2.7/site-packages/numpy/core/arrayprint.py", line 328, in _array2string
      _summaryEdgeItems, summary_insert)[:-1]
    File "/home/venv/lib/python2.7/site-packages/numpy/core/arrayprint.py", line 500, in _formatArray
      word = format_function(a[-1])
    File "/home/venv/lib/python2.7/site-packages/numpy/core/arrayprint.py", line 741, in __call__
      casting=self.casting)
  TypeError: Invalid datetime unit "generic" in metadata
  </code></pre>
  
  <p>If I let the code run without the print, I get this:</p>
  
  <pre><code>  File "/home/venv/lib/python2.7/site-packages/dat/array_utils.py", line 487, in set_to_nan
      arr[:] = _dtype_nan_value(arr.dtype)
  ValueError: Cannot convert from specific units to generic units in NumPy datetimes or timedeltas
  </code></pre>