Writing a date filter in a measure

Hi, mzienowicz99 

According to your description, I think [activity_month] column is a date column or a string column.

If it is a date column, you can use [activity_month]=date(2021,10,1). If it is a string column, you can use [activity_month]="2021-10".

Like this:

YTY Variance =
CALCULATE (
    CALCULATE (
        [QTY],
        R2_INDUSTRY[BRAND] = "JOHN DEERE",
        R2_INDUSTRY[ACTIVITY_MONTH] = "2021 - 10" //R2_INDUSTRY[ACTIVITY_MONTH] = date(2021,10,1)
    )
        - (
            CALCULATE (
                [QTY],
                R2_INDUSTRY[BRAND] = "JOHN DEERE",
                R2_INDUSTRY[ACTIVITY_MONTH] = "2020 - 10" //R2_INDUSTRY[ACTIVITY_MONTH] = date(2020,10,1)
            )
        )
)

Did I answer your question ? Please mark my reply as solution. Thank you very much.
If not, please feel free to ask me.

Best Regards,
Community Support Team _ Janey