title: How to cut yearly time-based data into 36 parts with R?
body: <p>I have a df like the following with 30 years until 2015. I want to cut every month into three data like 1-10, 11-20, and 21-31 and average all ten (less then ten) data.  Thus, each month has three data. How can I do it?</p>
  
  <pre><code>1993-01-29 28.92189
  1993-02-01 29.12760
  1993-02-02 29.18927
  1993-02-03 29.49786
  1993-02-04 29.62128
  1993-02-05 29.60068
  1993-02-08 29.60068
  1993-02-09 29.39498
  ------
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  2015-08-18 209.92999
  2015-08-19 208.28000
  2015-08-20 204.01000
  2015-08-21 197.63001
  2015-08-24 189.55000
  2015-08-25 187.23000
  2015-08-26 194.67999
  2015-08-27 199.16000
  2015-08-28 199.24000
  </code></pre>