title: Python: Iterate Over Year and Month in DatetimeIndex
body: <p>I have two DataFrames:</p>
  <pre><code>df1:
                    A    B 
  Date
  01/01/2020        2    4
  02/01/2020        6    8
  
  
  df2:
                    A    B
  Date
  01/01/2020        5    10
  </code></pre>
  <p>I want to get the following:</p>
  <pre><code>df3:
                    A    B 
  Date
  01/01/2020        10   40
  02/01/2020        30   80
  </code></pre>
  <p>What I want is to multiply the column entries based on year and month in DatetimeIndex. But I'm not sure how to iterate over datetime.</p>