title: Adding dates and zero padding a time series in Pandas?
body: <p>I have a time series dataframe that looks like this:</p>
  
  <pre><code>      Date     Var_1  Var_2  Var_3
  ------------------------------------
  0   01/01/14    1      1       2  
  1   01/08/14    2      1       3
  2   01/15/14    2      1       3
  3   01/22/14    1      0       3
  4   01/29/14    3      0       2 
  </code></pre>
  
  <p>I want to extend the dates by a few weeks prior to the start date and zero pad the other variables, so that my dataframe will look like: </p>
  
  <pre><code>      Date     Var_1  Var_2  Var_3
  ------------------------------------
  0   12/11/13    0      0       0  
  1   12/18/13    0      0       0
  2   12/25/13    0      0       0
  3   01/01/13    1      1       2  
  4   01/08/14    2      1       3
  5   01/15/14    2      1       3
  6   01/22/14    1      0       3
  7   01/29/14    3      0       2 
  </code></pre>
  
  <p>Are there any built in functions in Pandas to do this, or do I to create a separate dataframe with the additional date sequence and zeros in all the other columns and then concatenate the two? </p>