DAX: Date Difference required with New Measure

Daydiff =
VAR Created =
    MAX ( 'Dates'[CreatedDt] )
VAR Closed =
    MAX ( 'Dates'[ClosedDt] )
VAR NoOfDays =
    IF (
        Closed > Created,
        DATEDIFF ( Created, Closed, DAY ),
        DATEDIFF ( Closed, Created, DAY )
    )
RETURN
    IF ( NoOfDays >= 21, "Met SLA", "NotMet SLA" )

Just created another variable called NoOfDays with the calculation you wanted, then use it in the other if statement

ssvr, this should do it

DDiff_DueDateCloDate = 
VAR Due = INT( MAX ( 'Task'[DueDate] ) )
VAR Clos = INT( MAX ( 'Task'[ClosedDate] ) )
RETURN 
Due - Clos

INT() will convert a date into an integer.

Then you simply subtract one number from the other.  This will make the result show a negative number, and should be the fastest performance-wise.