Calculating the difference between a specific month, and the following months

Hi@ Ghaston

After my research by your new file,You can use this measure

Column 2 = 
    CALCULATE(
        SUM(
            Data[Val.stk va]),
            FILTER(
                Data,
                Data[Groupe marchandises]=EARLIER(Data[Groupe marchandises])&&Data[Year]=EARLIER(Data[Year])&&Data[Month]=EARLIER(Data[Month])))-



CALCULATE(
                    SUM(
                        Data[Val.stk va]),
                        FILTER(
                            Data,
                            Data[Groupe marchandises]=EARLIER(Data[Groupe marchandises])&&Data[Year]+1=EARLIER(Data[Year])&&Data[Month]=12))

Result:

Here is demo, please try it

https://www.dropbox.com/s/oe52n3yaehet74z/CSP%20-%20Cartographie%20stock%20hors%20PF%20-%20MSLB%20New.pbix?dl=0

Best Regards,

Lin