title: What does -0400 mean in Timestamp('2014-09-19 12:21:21.747000-0400')?
body: <p>I converted a series to EST with:</p>
  
  <pre><code>pd.to_datetime(my_series, unit='ms').apply(lambda x: x.tz_localize('UTC').tz_convert('US/Eastern'))
  </code></pre>
  
  <p>and the conversion, e.g. for one of the entries was:</p>
  
  <ul>
  <li><strong>Input:</strong> <code>1411143681747</code> </li>
  <li><strong>Output:</strong> <code>Timestamp('2014-09-19 12:21:21.747000-0400', tz='US/Eastern')</code></li>
  </ul>
  
  <p>What does <code>-0400</code> mean here?</p>