title: Replace DateTime column value where date is greater than other date
body: <p>I am attempting to replace all DateTime values in a certain column where the date is greater than a particular date. </p>
  
  <p>An example of the <code>df</code> is shown below. </p>
  
  <pre><code>date
  2016-01-03
  2016-01-03
  2016-01-03
  2016-01-02
  2016-01-02
  2016-01-02
  2016-01-01
  2016-01-01
  2015-12-31
  2015-12-31
  2015-12-31
  </code></pre>
  
  <p>I would like all dates to greater than <code>2016-01-01</code> to appear as <code>2016-01-01</code>.</p>
  
  <p>My desired expected output is shown below.</p>
  
  <pre><code>date
  2016-01-01
  2016-01-01
  2016-01-01
  2016-01-01
  2016-01-01
  2016-01-01
  2016-01-01
  2016-01-01
  2015-12-31
  2015-12-31
  2015-12-31
  </code></pre>
  
  <p>Any assistance that anyone could provide would be greatly appreciated.  </p>