title: How to convert UTC timestamp string to pandas datetime?
body: <p>I have a panda dataframe with a column with date format as below:</p>
  
  <pre><code>PublishDateTime= 2018-08-31 12:00:00-UTC
  </code></pre>
  
  <p>I used panda to_gbq() function to dump data into a bigquery table. Before dumping data, I make sure that the format of columns match with table scheme. publishedate is timestamp in bigquery table. How can achieve something similar to:</p>
  
  <pre><code>df['PublishDateTime'] = df['PublishDateTime'].astype('?????')
  </code></pre>
  
  <p>I tried <code>datetime[ns]</code> but that didn't work!</p>