title: spark scala dataframe timestamp conversion sorting?
body: <p>I have a csv of the form:</p>
  
  <pre><code>t,value
  2012-01-12 12:30:00,4
  2012-01-12 12:45:00,3
  2012-01-12 12:00:00,12
  2012-01-12 12:15:00,13
  2012-01-12 13:00:00,7
  </code></pre>
  
  <p>I convert that into dataframe using <a href="https://github.com/databricks/spark-csv]" rel="nofollow">spark-csv</a>. (so <code>t</code> is in <code>String</code> type and <code>value</code> is in Integer type). What's the appropriate spark scala way so the output is sorted by time?</p>
  
  <p>I was thinking to convert <code>t</code> to certain type which can allow dataframe <code>sortBy</code>. But I am not familiar which timestamp type allow dataframe sorting by time.</p>