title: computing missing months in timeseries
body: <p>I have a dataset in R that contains monthly values.  However, some months are missing.  For example:</p>
  
  <p>"2001-09-01" "2001-10-01" "2001-12-01" "2002-02-01"</p>
  
  <p>Nov 2001 is missing and Jan 2002 is missing.  How do I include those months into the timeseries and add a value of 0 ?</p>
  
  <p>Thanks!</p>