Calculate date filter

Anonymous 
Please try

Yield Mech. assemblyprevmonth =
VAR CurrentYearMonth =
    MAX ( 'Date'[Year Month Number] )
VAR MechAssembly =
    CALCULATETABLE (
        TJob,
        TJob[SekundærNavn] = "Mech. assembly",
        'Date'[Year Month Number] = CurrentYearMonth - 1,
        ALL ( 'Date' )
    )
RETURN
    ROUND (
        DIVIDE (
            SUMX ( MechAssembly, TJob[FirstPassQty] ),
            SUMX ( MechAssembly, TJob[Antal] )
        ),
        4
    )