title: Change Date Format from %y-%m-%d %h:%m:%s to %Y%M%D with lubridate and mutate
body: <p>I've got a <code>tbl_df</code>with two columns <em>StartTime</em> and <em>StopTime</em>. Both are <code>dttm</code>.</p>
  
  <p>I want to change its format from <code>"%y-%m-%d %h:%m:%s"</code> to <code>"%y%m%d"</code>. </p>
  
  <p>I've tried  </p>
  
  <pre><code>data &lt;- mutate(data, StartTime = ymd(StartTime), StopTime = ymd(StopTime))
  </code></pre>
  
  <p>But it returns</p>
  
  <blockquote>
    <blockquote>
      <p>Warning messages: 1: All formats failed to parse. No formats found. 2: All formats failed to parse. No formats found. </p>
    </blockquote>
  </blockquote>
  
  <p>How can I do it?</p>
  
  <p><em>Please, don't send other questions that don't use lubridate package.</em></p>
  
  <p>Thanks</p>