I have a problem with my panel data ( which is organised by month and municipality) were I want to take the difference between a pair of month from two different years, ex the observation for jan 2020 would be jan2020-jan2019 and the year before would imply jan2019-jan2018. This will be done for all month in my sample starting from May 2018 and going on to May 2020. I have played around with "egen" but don't really see how I can solve it. Any ideas how I could fix it?