I am new to R and I am working on a project that requires me to calculated the difference in new cases two weeks from the date that a mask mandate was enforced. My dataset consist of state, date (%y%m%d), mask (yes/no), daily new cases. Therefore, if mask = yes, then I need to calculated the difference in new cases two weeks from the date that the mandate was placed. I am not sure how to code this, much help will be appreciated.
This is a example of a portion of the dataset. It is grouped by state.
  Mask date       state new_case
  NA   2020-07-15  AL    1236
  NA   2020-07-16  AL    1254
  NA   2020-07-17  AL    1579
  Yes  2020-07-18  AL    2003
  NA   2020-07-19  AL    1745
  NA   2020-07-20  AL    1945
  NA   2020-07-21  AL    2114
  NA   2020-07-22  AL    1819
  NA   2020-07-23  AL    1126
  NA   2020-07-24  AL    1282
 
    