title: xarray/datetime64[ns]: remove or normalise time from datetime
body: <p>I have a data array <code>arr</code> with coordinate 'time'.
  <code>arr</code>: </p>
  
  <pre><code>    &lt;xarray.DataArray 'T' (time: 731)&gt;
  array([244.40161, 244.39998, ..., 244.40936, 244.40549], dtype=float32)
  Coordinates:
    * time         (time) datetime64[ns] 1979-01-01T09:00:00 ... 1980-12-31T09:00:00
  </code></pre>
  
  <p>Extracting the first 5 time coordinates,  <code>arr.time.values[:5]</code>: </p>
  
  <pre><code>array(['1979-01-01T09:00:00.000000000', '1979-01-02T09:00:00.000000000',
         '1979-01-03T09:00:00.000000000', '1979-01-04T09:00:00.000000000',
         '1979-01-05T09:00:00.000000000'], dtype='datetime64[ns]')
  </code></pre>
  
  <p>I want the format of my date-time to just be <code>'1979-01-01'</code>, <code>'1979-01-02'</code> etc. without the time, or normalise the time at 00:00:00.</p>
  
  <p>There are some solutions for pandas data frame but I'm not quite sure how to apply them here since the functions aren't applicable (<a href="https://stackoverflow.com/questions/13703720/converting-between-datetime-timestamp-and-datetime64">Converting between datetime, Timestamp and datetime64</a>, <a href="https://stackoverflow.com/questions/16176996/keep-only-date-part-when-using-pandas-to-datetime">Keep only date part when using pandas.to_datetime</a>)</p>