Calculation based on AND of selected filter values

Hello Brysonds 

If I am understanding correctly this is fairly straight forward with a date table that contains a column with the year and month combined into a single number.  This DAX code will give us a simple date table to use.  Enter it in modeling > new table.

Dates = 
VAR DateRange = CALENDARAUTO()

RETURN 
ADDCOLUMNS(
    DateRange,
    "Year",YEAR([Date]),
    "Month",FORMAT([Date],"mmmm"),
    "Year Month", FORMAT([Date],"yyyy-mmmm"),
    "YearMonthSort",YEAR([Date])*100 + MONTH([Date]),
    "ShortName",FORMAT([Date],"ddd"),
    "IsWeekDay", NOT WEEKDAY( [Date] ) IN {1,7}
)

You would join this into your model by linking Dates[Date] > 'ABCD Customer YTD'[Period].

We will want a measure that just sums the amount.

Sales Amount = SUM('ABCD Customer YTD'[(YTD) Sales Amount])

Then this measure will calculate the lowest Year Month and the highest year month and give us the difference.

Difference = 
VAR FirstMonth = MIN ( 'Dates'[YearMonthSort] )
VAR LastMonth = MAX ( 'Dates'[YearMonthSort] )
VAR FirstMonthAmt = CALCULATE( [Sales Amount], 'Dates'[YearMonthSort] = FirstMonth )
VAR LastMonthAmt = CALCULATE( [Sales Amount], 'Dates'[YearMonthSort] = LastMonth )
RETURN
FirstMonthAmt - LastMonthAmt