Finding the difference between date values in the same row of different column within a Matrix

Joe_Barry Thanks for your contribution on this thread.

Hi Merle ,

You can create a measure as below to get it, please find the details in the attachment.

WOW = 
VAR _pcdate =
    SELECTEDVALUE ( 'Table'[Purchase Complete] )
VAR _category =
    SELECTEDVALUE ( 'Table'[Category] )
VAR _predate =
    CALCULATE (
        MAX ( 'Table'[Purchase Complete] ),
        FILTER (
            ALLSELECTED ( 'Table' ),
            'Table'[Category] = _category
                && 'Table'[Purchase Complete] < _pcdate
        )
    )
VAR _prevalue =
    CALCULATE (
        SUM ( 'Table'[Sales] ),
        FILTER (
            ALLSELECTED ( 'Table' ),
            'Table'[Category] = _category
                && 'Table'[Purchase Complete] = _predate
        )
    )
RETURN
    DIVIDE ( SUM ( 'Table'[Sales] ) - _prevalue, _prevalue )  

Best Regards