title: python linear regression date as axis
body: <p>i'm getting data from mysql and using panda DataFrame i'm separating the data to column </p>
  
  <pre><code>data = pd.DataFrame(data)
  print(data.ix[:,3])
  
  0     2006-04-01
  1     2006-08-01
  2     2006-12-01
  3     2006-02-01
  4     2006-01-01
  5     2006-07-01
  6     2006-06-01
  7     2006-03-01
  8     2006-05-01
  9     2006-11-01
  10    2006-10-01
  11    2006-09-01
  12    2007-04-01
  13    2007-08-01
  14    2007-12-01
  15    2007-02-01
  16    2007-01-01
  17    2007-07-01
  18    2007-06-01
  19    2007-03-01
  20    2007-05-01
  21    2007-11-01
  22    2007-10-01
  23    2007-09-01
  24    2009-04-01
  25    2009-08-01
  </code></pre>
  
  <p>when i put this data into linear regression prediction it gives dtype is different. how can i convert this date field to int and put it in the linear prediction </p>