title: Pandas dataframe reduce row values between specific dates
body: <pre><code>datetime       JD
  2000-01-01      1
  2000-01-02      2
  2000-01-03      3
  2000-01-04      4
  2000-01-05      5
  2000-01-06      6
  2000-01-07      7
  2000-01-08      8
  2000-01-09      9
  </code></pre>
  
  <p>I have the dtaaframe above, where the index is the column datetime. I would like to decrease the values in the column JD (by 1) from 5th Jan 2000 to 8th Jan 2000, using the dates (and not merely row numbers). Is there a pandas command to do this? I have been playing around with apply, but not sure how to use it</p>