title: Find a one year date from now in Python/Pandas with approximations
body: <p>I have a df pandas with in column just a 'Price' and in index dates. I want to find a new column called 'Aprox' with inside</p>
  
  <pre><code> aprox. = price of today - price of one year ago (or closest date from a year ago) - 
  price in one year (again take aprox if exact one year price don't exist)
   for example   
   aprox. 2019-04-30 = 8 -4 -10 = -6 = aprox. 2019-04-30
                                              - aprox. 2018-01-31 - aprox.2020-07-30  
  </code></pre>
  
  <p>To be honest I am a bit strugling with that... </p>
  
  <pre><code>ex. [in]:      Price
  2018-01-31       4  
  2019-04-30       8 
  2020-07-30       10   
  2020-10-31       9  
  2021-01-31       14   
  2021-04-30       150
  2021-07-30       20
  2022-10-31       14
  
     [out]:      Price    aprox.
  2018-01-31       4  
  2019-04-30       8       -6  ((8-4-10) = -6) since there is no 2018-04-30 
  2020-07-30       10      -12 (10-14-8)
  2020-10-31       9       ...
  2021-01-31       14      ...
  2021-04-30       150
  2021-07-30       20
  2022-10-31       14
  </code></pre>
  
  <p>I am strugling very much with that... even more with the approx. </p>
  
  <p>Thank you very much!!</p>