title: pandas multi-index of integers to datetime index?
body: <p>I have a dataframe like the following, with a multi-index of integers that represents months and days of the year, along with maximum and minimum temperature recordings from those days.</p>
  
  <pre><code>df
  
            Min Temp  Max Temp
  Date Date                    
  1    1          -88       139
       2         -115       150
       3         -110       139
       4          -81       156
       5          -80       172
  ...             ...       ...
  12   2          -94       156
       3          -97       172
       4         -120       156
       5         -124       144
       6         -161       130
       7         -167       135
       8         -141       167
       9         -135       178
       10        -106       194
       11        -106       161
       12         -94       144
       13         -92       133
       14        -149       117
       15        -158       117
       16        -119       122
       17        -111       160
       18        -142       133
       19        -185       130
       20        -190       161
       21        -167       161
       22         -98       150
       23        -162       139
       24         -90       183
       25        -125       183
       26        -119       144
       27         -76       130
       28         -81       134
       29        -117       113
       30        -127       106
       31        -111       122
  </code></pre>
  
  <p>How can I convert this multi-index to a single index that is of type datetime? Something like this conversion is what I am looking for:</p>
  
  <pre><code>1 1 ---&gt; January 1
  1 2 ---&gt; January 2
  ...
  12 31 ---&gt; December 31
  </code></pre>