title: Faster solution for date formatting
body: <p>I am trying to change the format of the date in a pandas dataframe. 
  If I check the date in the beginning, I have:</p>
  
  <pre><code>df['Date'][0]
  Out[158]: '01/02/2008'
  </code></pre>
  
  <p>Then, I use:</p>
  
  <pre><code>df['Date'] = pd.to_datetime(df['Date']).dt.date  
  </code></pre>
  
  <p>To change the format to </p>
  
  <pre><code>df['Date'][0]
  Out[157]: datetime.date(2008, 1, 2)
  </code></pre>
  
  <p>However, this takes a veeeery long time, since my dataframe has millions of rows. </p>
  
  <p>All I want to do is change the date format from MM-DD-YYYY to YYYY-MM-DD.
  How can I do it in a faster way?</p>