title: Merge date and time columns
body: <p>Currently I have 2 columns one that holds dates and the other holds the time (24hr text version). </p>
  
  <p>Example:</p>
  
  <pre><code>a_dt          a_time_text
  01-Jan-15     0224
  31-Mar-15     0317
  15-Sep-15     2010
  </code></pre>
  
  <p>How do I create a new column that hold the date and time, like this:</p>
  
  <pre><code>a_dt_tm
  01-Jan-15 02:24
  31-Mar-15 03:17
  15-Sep-15 20:10
  </code></pre>
  
  <p>Or any other way so that I can find the difference in date/time between another variable.</p>