How can I calculate the TTM and T3M change between columns for sales differences.

[jberistain]

I have sales by item and customer by month in a fiscal period format (202301). I want to calculate the chance for each month for these two metrics and flag each row that has a drop of 50% or more on both metrics. I want to then do the sales from previous month - sales of that current month to find out how much we dropped on a month-to-month basis. 

I am hoping to compare the previous twelve months vs the previous twelve months before that. so if we are in 202304 I would do TTM = 202204 thru 202303 and then PTTM 202203 thru 202104 and the final calc would be PTTM - TTM. and same thing for the T3M

Thank you.

[jberistain]

I am hoping to compare the previous twelve months vs the previous twelve months before that. so if we are in 202304 I would do TTM = 202204 thru 202303 and then PTTM 202203 thru 202104 and the final calc would be PTTM - TTM. and same thing for the T3M