title: How can I convert my datetime column in pandas all to the same timezone
body: <p>I have a dataframe with a DataTime column (with Timezone in different formats). It appears like timezone is UTC but I want to convert the column to <code>pd.to_datetime</code> and that is failing. That is problem #1. Since that fails I cannot do any datetime operations on the time period such as group the column by date / figure out the days / group by hour of the day and so on. Here's my dataframe <code>df_res</code></p>
  
  <pre><code>    DateTime
      2017-11-02 19:49:28-07:00
      2017-11-27 07:32:22-08:00
      2017-12-27 17:01:15-08:00
  </code></pre>
  
  <p>OUTPUT for the command </p>
  
  <pre><code>      df_res["DateTime"] = df_res["DateTime"].dt.tz_convert('America/New_York')
  </code></pre>
  
  <p><code>AttributeError: Can only use .dt accessor with datetimelike values</code></p>
  
  <p>WHen I convert to <code>datetime</code> </p>
  
  <pre><code>   df_res['DateTime'] = pd.to_datetime(df_res['DateTime'])
  </code></pre>
  
  <p><code>ValueError: Tz-aware datetime.datetime cannot be converted to datetime64 unless utc=True</code></p>
  
  <p>I feel I am going around in circles. I need to convert the column to datetime in order to perform operations &amp; in order to do that I need to have them all the same timezone but I cannot have the same timezone unless it is a datetime object so how can I best approach this. 
  I did refer to previous postings but they seem to convert to datetime as easily as possible: </p>
  
  <p><a href="https://stackoverflow.com/questions/34789888/convert-datetime-columns-to-a-different-timezone-pandas">Convert datetime columns to a different timezone pandas</a>
  <a href="https://stackoverflow.com/questions/16628819/convert-pandas-timezone-aware-datetimeindex-to-naive-timestamp-but-in-certain-t/34687479#34687479">Convert pandas timezone-aware DateTimeIndex to naive timestamp, but in certain timezone</a></p>