Find the different before sum up

I have a measure that pwerform sum up as shown.

.
I would like to have additional 2 measures:-

1) to calculate the difference between the values of these two dates before summing them up. i.e. this month values - last month values then perform the sum up.

2) to calculate how many values that was <>0 (<0 or >0) after finding the dfferent.

How can I achieve this? Thanks.

Sum QR-A = 
VAR Selected_Date = MAX(CalendarDate[Date])
VAR Previous_Date = CALCULATE(MAX(CalendarDate[Date]), PREVIOUSMONTH(CalendarDate[Date]))

VAR Sum_A = CALCULATE(
                    SUM('Raw Data'[Gradient-A]), 
                    FILTER(
                        'Raw Data',
                        NOT(
                            (
                                YEAR('Raw Data'[P-Date]) = YEAR(Selected_Date) && 
                                MONTH('Raw Data'[P-Date]) = MONTH(Selected_Date)
                            ) ||
                            (
                                YEAR('Raw Data'[R-Date]) = YEAR(Selected_Date) && 
                                MONTH('Raw Data'[R-Date]) = MONTH(Selected_Date)
                            ) ||
                            (
                                YEAR('Raw Data'[P-Date]) = YEAR(Previous_Date) && 
                                MONTH('Raw Data'[P-Date]) = MONTH(Previous_Date)
                            ) ||
                            (
                                YEAR('Raw Data'[R-Date]) = YEAR(Previous_Date) && 
                                MONTH('Raw Data'[R-Date]) = MONTH(Previous_Date)
                            )
                        )
                    )
                )

RETURN Sum_A + 0