title: Combine Pandas DataFrame DateTime Columns
body: <p>Supposely I have dataframes as below:</p>
  
  <pre><code>Year Month Day
  2003 1     8
  2003 2     7
  </code></pre>
  
  <p>How to combine the Year, Month, and Day in the newly defined column in the dataframe as such the dataframe would be:</p>
  
  <pre><code>Year Month Day Date
  2003 1     8   2003-1-8
  2003 2     7   2003-2-7
  </code></pre>
  
  <p>Any idea on this?</p>
  
  <p>I am using pandas python dataframe</p>
  
  <p>Thanks!</p>