title: Python Pandas sum a constant value in Columns If date between 2 dates
body: <p>Let's assume a dataframe using datetimes as index, where we have a column named 'Score', initialy set to 10:</p>
  
  <pre><code>            score
  2016-01-01  10
  2016-01-02  10
  2016-01-03  10
  2016-01-04  10
  2016-01-05  10
  2016-01-06  10
  2016-01-07  10
  2016-01-08  10
  </code></pre>
  
  <p>I want to substract a fixed value (let's say 1) from the score, but only when the index is between certain dates (for example between the 3rd and the 6th):</p>
  
  <pre><code>            score
  2016-01-01  10
  2016-01-02  10
  2016-01-03  9
  2016-01-04  9
  2016-01-05  9
  2016-01-06  9
  2016-01-07  10
  2016-01-08  10
  </code></pre>
  
  <p>Since my real dataframe is big, and I will be doing this for different dateranges and different fixed values N for each one of them, I'd like to achieve this without requiring to create a new column set to -N for each case.</p>
  
  <p>Something like numpy's <code>where</code> function, but for a certain range, and allowing me to sum/substract to current value if the condition is met, and do nothing otherwise. Is there something like that?</p>