% growth over PY ignore blank

[Credo]

Hi

I have a formula for sales growth over PY 

% Sales Growth over PY =
DIVIDE (
    SUM ( 'FactSales'[Sales] );
    CALCULATE (
        SUM ( 'FactSales'[Sales] );
        SAMEPERIODLASTYEAR ( 'DimCalendar'[Dates] )
    )
) -1

My data only starts in April 2017. 

The current formula has limitations related to that

For example if on date range slicer:

May 2018 selected  =                                                                  (-2.6%)

April and May 2018 selected =                                                   (-1.6%)

March (blank in 2017), April and May 2018 selected  =            +50,4% 

How can I ignore values in the selected month if the PY month has no data? 

Thanks in advance for your help

[v-shex-msft · Accepted solution]

HI @Credo,

I'd like to suggest you add a condition to compare 'previous date' and minimum date, calculate result when minimum date is 'large or equal to' previous date.

% Sales Growth over PY =
VAR currDate =
    MAX ( 'DimCalendar'[Dates] )
VAR minDate =
    FIRSTDATE ( ALLSELECTED ( 'DimCalendar'[Dates] ) )
RETURN
    IF (
        minDate
            <= DATE ( YEAR ( currDate ) - 1; MONTH ( currDate ); DAY ( currDate ) );
        DIVIDE (
            SUM ( 'FactSales'[Sales] );
            CALCULATE (
                SUM ( 'FactSales'[Sales] );
                SAMEPERIODLASTYEAR ( 'DimCalendar'[Dates] )
            )
        )
            - 1
    )

Regards,

Xiaoxin Sheng