title: How to convert multi-indexed datetime index into integer?
body: <p>I have a multi indexed dataframe(groupby object) as the result of groupby (by 'id' and 'date').</p>
  
  <pre><code>                x  y
  id  date            
  abc 3/1/1994  100  7
      9/1/1994   90  8
      3/1/1995   80  9
  bka 5/1/1993   50  8
      7/1/1993   40  9 
  
  </code></pre>
  
  <p>I'd like to convert those dates into an integer-like, such as</p>
  
  <pre><code>             x  y
  id  date            
  abc day 0  100  7
      day 1   90  8
      day 2   80  9
  bka day 0   50  8
      day 1   40  9 
  </code></pre>
  
  <p>I thought it would be simple but I couldn't get there easily. Is there a simple way to work on this?</p>