title: Averaging over specific months in pandas
body: <p>I'm having trouble creating averages using pandas. My problem is that I want to create the averages combining the months Nov,Dec,Jan,Feb,March, for each winter, however they fall on different years and therefore I can't just do an average of those values falling within one calendar year. I have tried subsetting the data into two datetime objects as..</p>
  
  <pre><code>nd_npi_obs = ndjfm_npi_obs[ndjfm_npi_obs.index.month.isin([11,12])]
  jfm_npi_obs = ndjfm_npi_obs[ndjfm_npi_obs.index.month.isin([1,2,3])]
  </code></pre>
  
  <p>..however I'm having trouble manipulating the dates (years) in order to do a simple average. I'm inexperienced with pandas and wondering if there is a more elegant way than exporting to excel and changing the year! The data is in the form.. </p>
  
  <pre><code>Date    
  1899-01-01 00:00:00 100994.0
  1899-02-01 00:00:00 100932.0
  1899-03-01 00:00:00 100978.0
  1899-11-01 00:00:00 100274.0
  1899-12-01 00:00:00 100737.0
  1900-01-01  100655.0
  1900-02-01  100633.0 
  1900-03-01  100512.0
  1900-11-01  101212.0
  1900-12-01  100430.0
  </code></pre>