Convert Accounting Period to Date in Excel

I have a report that is exported out of our ERP system. Instead of providing Month and Year to reflect the months usage for that product (PS NUM). I would like to convert the "ACCTG PER" to the appropriate month and year. The trick is that "0" should equal current month and year, "1" should equal previous month and year, and so on(See "Ideal result"). The reason I want to have month and year is I would like to convert to a column and line graph so that the "Average" would be the line, and the columns would represent the actual usage of that month. The trick is that if I were to run this report next month in May. The "ACCTG PER" zero (0) would be 05/2024, and one (1) would be 04/2024. Whereas this current report 04/2024 is "ACCTG PER" (0). Also some products will not have a "0" accounting period as the system does not enter a "0" usage to the "ACCTG PER" until the month closes out. So a product can have no "ACCTG PER" for the entire month.

The trick after this would be to append the newly formatted report to a master historical file, so it would only add the new months data. Which would be to replace the "0" with the final numbers as the current months usage will continue to adjust until the new month.

I would add a slicer/ Filter to review an individual Product. As I want to identify any product which had a demand greater than 30% above the historical average, to include the opposite 30% below the average historical demand. As in the example below the two months that would be "flagged" would be November (23) and March (24).