title: How to get the next date avaliable in timeseries dataframe
body: <p>I have the following list of dates:</p>
  <pre><code>DatetimeIndex(['2015-11-19', '2015-12-17', '2015-12-23', '2016-01-19',
                 '2016-01-22', '2016-01-29', '2016-02-05', '2016-02-15',
                 '2016-02-18', '2016-03-01',
                 ...
                 '2020-07-17', '2020-07-20', '2020-07-21', '2020-07-28',
                 '2020-08-04', '2020-08-18', '2020-09-10', '2020-09-25',
                 '2020-09-28', '2020-10-01'],
                dtype='datetime64[ns]', name='Date', length=137, freq=None)
  </code></pre>
  <p>I got these dates from operations on <code>df</code> and I also have <code>df2</code> which also has these dates but the next date is not <code>timedelta(day=1)</code>. Thus for example the first date is <code>2015-11-19</code> but the next date could be <code>2015-11-22</code>. Since the dates are index values, how can i get the next available dates?</p>