title: Display missing timestamp given start and end date in python
body: <p>Dataset is having time and Temperature values. Start and end date is also given.</p>
  
  <pre><code>start_date = '22-02-2018 10:35:29'
  end_date = '23-02-2018 10:34:29'
  
  TIMESTAMP           Temp1   Temp2
  22-02-2018 14:35    4.34    4.93
  22-02-2018 14:36    4.35    5.02
  22-02-2018 14:37    4.35    5.1
  22-02-2018 14:39    4.31    5.23
  22-02-2018 14:40    4.29    5.26
  22-02-2018 14:41    4.26    5.24
  22-02-2018 14:42    4.24    5.17
  22-02-2018 14:47    4.09    4.64
  22-02-2018 14:48    4.08    4.55
  22-02-2018 14:49    4.08    4.48
  22-02-2018 14:50    4.09    4.48
  22-02-2018 14:51    4.11    4.5
  22-02-2018 14:54    4.22    4.66
  22-02-2018 14:55    4.25    4.72
  </code></pre>
  
  <p>Timestamps from the start and end date is missing. So I want to add timestamps from start data and end date corresponding to that NAN values in the temp1 and temp2 data. Frequency is <code>60S</code>. </p>
  
  <p>Desired result:</p>
  
  <pre><code>TIMESTAMP           Temp1 Temp2
  22-02-2018 10:35    NaN NaN
  22-02-2018 10:36    NaN NaN
  22-02-2018 10:37    NaN NaN
  22-02-2018 10:38    NaN NaN
  22-02-2018 10:39    NaN NaN
  22-02-2018 10:40    NaN NaN
  22-02-2018 10:41    NaN NaN
  22-02-2018 10:42    NaN NaN
  .
  .
  .
  22-02-2018 14:35    4.34    4.93
  22-02-2018 14:36    4.35    5.02
  22-02-2018 14:37    4.35    5.1
  22-02-2018 14:38    NaN     NaN
  22-02-2018 14:39    4.31    5.23
  22-02-2018 14:40    4.29    5.26
  22-02-2018 14:41    4.26    5.24
  22-02-2018 14:42    4.24    5.17
  22-02-2018 14:43    NaN NaN
  22-02-2018 14:44    NaN NaN
  22-02-2018 14:45    NaN NaN
  22-02-2018 14:46    NaN NaN
  22-02-2018 14:47    4.09    4.64
  22-02-2018 14:48    4.08    4.55
  22-02-2018 14:49    4.08    4.48
  </code></pre>