Add Date Variable Filter to Calculate

I figured this out, and again it makes perfect sense once I wrap my head aroudn the logic of it.   The variables will (I think) always compute their values first before you get into the Calculate function.  So the 'trick' was to compute the number of days in the Switch Variable, and then pass that number into the Return/Calculate.

So this works and computes as expected.

Var MyPeriod = 
        SWITCH (
		    VALUES ('Measures List'[Fact Period]),
	    	    "Day", 1,
                "Last 7", 7,
		        "MTD", DATEDIFF( STARTOFMONTH('Dim Period'[Date]), MAX('Dim Period'[Date]), DAY ) + 1,
		        "QTD", DATEDIFF( STARTOFQUARTER('Dim Period'[Date]), MAX('Dim Period'[Date]), DAY ) + 1,
		        "YTD", DATEDIFF( STARTOFYEAR('Dim Period'[Date]), MAX('Dim Period'[Date]), DAY ) + 1 
	    )

    RETURN

        SWITCH (
            VALUES ('FactType'[FactType]),
                "#", CALCULATE(
                    [Qty]
                    , 'Dim Item','Dim Item'[description]=Myitem
                    ,  DATESINPERIOD ( 'Dim Period'[Date], MAX ( 'Dim Period'[Date] ), -VALUE(MyPeriod), DAY) // this works
                ),