title: Timestamp subtraction of time arrays with different timezones
body: <p>I have the following code from somebody else that has a similar problem, but the solution proposed does not work on my DataFrame.  The code subtracts a Pandas DataFrame index from a given date:</p>
  
  <pre><code>my_date = pd.datetime.today()
  MyDF['day_differential'] = (MyDF.index - my_date).days
  </code></pre>
  
  <p>Which is generating the following error in my DataFrame:</p>
  
  <pre><code>TypeError: Timestamp subtraction must have the same timezones or no timezones
  </code></pre>
  
  <p>How do I found out tz for both dates? How do I make them the same so I can calculate the number of days between them?</p>