title: Filtering dataframe based on date (keep only rows that occur 6 months after each other)
body: <p>I would like to ask you for help:
  I have a dataframe which consists of 2 columns: ID and Month.
  I would like to delete the rows where the ID is appearing before a 6 months time span.</p>
  <p>For example, if customer ID 1 appears in month March I want him to appear only after 6 months, for example in September or later and I want to delete the rows in which my customer ID 1 appears before the 6 months time, for example in April.</p>
  <p>Below an example of my dataset and the expected output.</p>
  <p><a href="https://i.stack.imgur.com/Umh6z.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Umh6z.png" alt="example-dataset-and-expected-output" /></a></p>