title: How to convert unix epoch time to datetime with timezone in pandas
body: <p>I have many csv files containing Unix epoch time which needs to be converted to human readable date/time. The following Python code does the job but it is very slow.</p>
  <pre><code>df['dt'] = pd.to_datetime(df['epoch'], unit='s')
  df['dt'] = df.apply(lambda x: x['dt'].tz_localize('UTC').tz_convert('Europe/Amsterdam'), axis=1)
  </code></pre>
  <p>Actually, the second line is the bottleneck (~30 seconds for 1 million rows). So even with the aid of multiprocessing, it is not scalable as I have more than a billion records totally. How can I make it faster?</p>