title: pandas to_json - return timestamp with units of days rather than seconds
body: <p>I have a pandas series containing datetime objects which have been created from day-month-year strings</p>
  
  <pre><code>series = pd.Series(['3/11/2000', '3/12/2000', '3/13/2000'])
  series = pd.to_datetime(series)
  
  print series
  0   2000-03-11
  1   2000-03-12
  2   2000-03-13
  dtype: datetime64[ns]
  </code></pre>
  
  <p>Later, after using these datetime objects, I want to convert this series into json in a format day-month-year. However, to_json returns the datetime with HH:MM:SS etc</p>
  
  <pre><code>json = series.to_json(orient='index', date_format='iso', date_unit = 's')
  print json
  {"0":"2000-03-11T00:00:00Z","1":"2000-03-12T00:00:00Z","2":"2000-03-13T00:00:00Z"}
  </code></pre>
  
  <p>Is there any inbuilt and elegant way to just return the dates as so</p>
  
  <pre><code>{"0":"2000-03-11","1":"2000-03-12","2":"2000-03-13"}
  </code></pre>
  
  <p>without the HH:MM:SS etc. The closest I have got (without converting to strings and writing a function to parse) is the date_unit argument of to_json although the largest time unit is seemingly seconds. </p>
  
  <p>Can anyone help? Thanks</p>