Calculating P12M data but excluding blank months

Hi joyceleeyw ,

One sample for your reference, please check the following steps as below.

1. Create a calculated column in the fact table.

YM = FORMAT('Table'[date],"yyyymmmm")

2. After that, we can create measures as below to get P12 or P6 average.

average p12M = 
VAR A =
    MAX ( 'Table'[date] )
VAR p12 =
    EDATE ( A, -12 )
RETURN
    DIVIDE (
        CALCULATE (
            SUM ( 'Table'[value] ),
            FILTER ( 'Table', 'Table'[date] >= p12 && 'Table'[date] <= A )
        ),
        CALCULATE (
            DISTINCTCOUNT ( 'Table'[YM] ),
            FILTER ( 'Table', 'Table'[date] >= p12 && 'Table'[date] <= A )
        )
    )

average p6M = 
VAR A =
    MAX ( 'Table'[date] )
VAR p12 =
    EDATE ( A, -6 )
RETURN
    DIVIDE (
        CALCULATE (
            SUM ( 'Table'[value] ),
            FILTER ( 'Table', 'Table'[date] >= p12 && 'Table'[date] <= A )
        ),
        CALCULATE (
            DISTINCTCOUNT ( 'Table'[YM] ),
            FILTER ( 'Table', 'Table'[date] >= p12 && 'Table'[date] <= A )
        )
    )

For more details, please check the pbix as attached.