Prevent Offset being filtered

Hi, 

I'm trying to create a measure for compared to same period last year (I can't use the normal time intelligence as we use 13 periods in the year).

I am using offset to get the value for 13 periods prior, which works perfectly until I try to filter the data - it blanks.

I've tried replacing the ALLSELECTED with ALL but then the OFFSET breaks.

CALCULATE(
    [% Accum YTD Joiners (Heads)],
    OFFSET(
        -13,
        ALLSELECTED('Reporting DimDate'[Short_RSP],'Reporting DimDate'[Short_RSPYear])
        ,ORDERBY('Reporting DimDate'[Short_RSPYear], ASC )
        )
)

I want to filter this by DimDate