title: How to correctly use fillna() for a datetime column in pandas dataframe (not working)?
body: <p>I have a dataframe as shown below:</p>
  
  <pre><code>       Age  CreatedDate   TestDate               Performance
  544    51   2015-11-23    2015-12-11                   1
  325    51   2015-11-23    2016-01-04                   0
  1043   51   2016-01-26    2016-01-25                   1
  303    51   2016-01-26    2016-02-15                   1
  1076   50   2015-04-29           NaT                   0
  </code></pre>
  
  <p>I want to update my TestDate which has value of NaT with the data from Created date, as such:</p>
  
  <pre><code>       Age  CreatedDate   TestDate               Performance
  544    51   2015-11-23    2015-12-11                   1
  325    51   2015-11-23    2016-01-04                   0
  1043   51   2016-01-26    2016-01-25                   1
  303    51   2016-01-26    2016-02-15                   1
  1076   50   2015-04-29    2015-04-29                   0
  </code></pre>
  
  <p>I've tried to use the method fillna(), however, my data is not being updated though there is no error emitted.</p>
  
  <pre><code>df['TestDate'].fillna(pd.to_datetime(df['CreatedDate']))
  </code></pre>
  
  <p>Any advice?
  Thanks Before.</p>