title: How to get the cumulative sum of n days in different months?
body: <p>I have this df:</p>
  <pre><code>       CODE      DATE     PP
  17594  000130 1991-01-01  0.5
  17595  000130 1991-01-02  11
  17596  000130 1991-01-03  1
  17597  000130 1991-01-04  2
  17598  000130 1991-01-05  5
  17599  000130 1991-01-06  2
  17598  000130 1991-01-07  5
  17598  000130 1991-01-08  7
  17598  000130 1991-01-09  5
  17598  000130 1991-01-10  1
  17598  000130 1991-01-11  5
  17598  000130 1991-01-12  1
  17598  000130 1991-01-13  5
  17598  000130 1991-01-14  1
  17598  000130 1991-01-15  5
  17598  000130 1991-01-16  3
  17598  000130 1991-01-17  5
  17598  000130 1991-01-18  3
  17598  000130 1991-01-19  5
  17598  000130 1991-01-20  3
  17598  000130 1991-01-21  5
  17598  000130 1991-01-22  2
  17598  000130 1991-01-23  5
  17598  000130 1991-01-24  2
  17598  000130 1991-01-25  5
  17598  000130 1991-01-26  1
  17598  000130 1991-01-27  2
  17598  000130 1991-01-28  5
  17598  000130 1991-01-29  3
  17598  000130 1991-01-30  5
  17598  000130 1991-01-31  4
  ...    ...    ...         ...
  28455  000130 1991-12-27  10
  28456  000130 1991-12-28  12
  28457  000130 1991-12-29  0
  28458  000130 1991-12-30  0
  28459  000130 1991-12-31  0
  </code></pre>
  <p>I want to get the cumulative sum of 10 days (3 per month) for the PP column. I'll give you and example:</p>
  <p>For the month of April: 04-01 to 04-10, 04-11 to 04-20 and 04-21 to 04-30.</p>
  <p>If the month has 31 days, eg January: 01-21 to 01-31 (11 days in the last cumulative sum).</p>
  <p>If it has 28 days like February, from 02-21 to 02-28 (just 8 days in the last cumulative sum) and if it is a leap year, from 02-21 to 02-29 (9 days in the last cumulative sum).</p>
  <p>Expected result:</p>
  <pre><code>       CODE      DATE     PP
  17594  000130 1991-01-01  39.5
  17595  000130 1991-01-11  36
  17596  000130 1991-01-21  39
  17597  000130 1991-02-01  27
  17598  000130 1991-02-11  30
  17598  000130 1991-02-21  18
        ...        ...  ...
  28455  000130 1991-11-01  22
  28456  000130 1991-11-11  21
  28457  000130 1991-11-21  12
  28458  000130 1991-12-01  11
  28459  000130 1991-12-11  10
  28460  000130 1991-12-21  25
  </code></pre>
  <p>I tried this code but it sums the 10 day with no difference between the months with 31, 30, 29 and 28 days.</p>
  <pre><code>cumulative=df['PP'].resample('10D').sum().round(1)
  </code></pre>
  <p>Would you mind to help me?</p>
  <p>Thanks in advance.</p>