title: extracting hour and minute from character column in r
body: <p>I have the following data frame,the data set is already imported from a database table and created_at column has character type:</p>
  
  <pre><code>sale_id      created_at
  1               2016-05-28T05:53:31.042Z
  2               2016-05-30T12:50:58.184Z
  3               2016-05-23T10:22:18.858Z
  4               2016-05-27T09:20:15.158Z
  5               2016-05-21T08:30:17.337Z
  6               2016-05-28T07:41:14.361Z
  </code></pre>
  
  <p>How can i extract only hour and minute from created_at column , preferably using base r libraries? i need to paste  hour and minute together later and put it as a new column.</p>