title: Get rid of NaT values from pandas dataframe
body: <p>I have a dataframe that looks like shown below</p>
  
  <pre><code>                               mean
  comp_name  date                      
  Appdynamics 2012-05-01 00:18:15.910000
              2012-05-01             NaT
              2012-05-01             NaT
              2012-05-02 00:20:12.145200
              2012-05-02             NaT
              2012-05-02             NaT
  </code></pre>
  
  <p>Here the comp_name and date form multiindex. I want to get rid of the NaT values and obtain only those rows where the mean(timedelta64) is not NaT.</p>
  
  <pre><code>                               mean
  comp_name  date                      
  Appdynamics 2012-05-01 00:18:15.910000
              2012-05-02 00:20:12.145200
  </code></pre>
  
  <p>Any ideas on this?</p>