title: Take differences from one dataframe based on dates from another dataframe
body: <h1>Dataset A</h1>
  <pre><code>Name  Date    Value  change  year NewColumn
  A     2000-02   0.5  NA      2000        NA
  A     2001-03   0.4  -0.200  2001         0
  A     2002-02   1     1.5    2002         1
  A     2003-05   0.9  -0.100  2003         0
  A     2004-06   0.9   0      2004        NA
  A     2006-03   0.4  -0.556  2006        NA
  </code></pre>
  <p>So basicalley I want to look at the column date if there is a (0/1) in my column NewColumn and then look at the dates from this year and the previous year from my dataset A and take based on that the values from the same dates but one month earlier from the dataset B and take put the difference in my new dataset.</p>
  <p>Is there a way of coding this easily?</p>
  <p>If not, I guess I have to do it manually by mself using excel.
  Or is there a less timecomsuming way or trick I could handle this problem :)</p>
  <p>Thank you :)</p>