title: Pandas: number of days elapsed since a certain date
body: <p>I have a dataframe with a 'date' column with ~200 elements in the format yyyy-mm-dd.</p>
  
  <p>I want to compute the number of days elapsed since 2001-11-25 for each of those elements and add a column of those numbers of elapsed days to the dataframe.</p>
  
  <p>I know of the to_datetime() function but can't figure out how to make this happen.</p>