Hope you are doing well. You can solve such type of problem with the use of parameters. Being more specefic, create a parameter of current date and a calculated feild that calculates sales of only the month taken from the parameter. Create another calculated feild that calculates sales of n-1 months selected from the paramter (excluding the selected month).
I hope this solves your purpose, you can try this method out and then attach a workbbok if you are getting stuck somewhere in the solution.