First Dax attempt, not working.

[tazeo]

I'm trying to create a visualisation that goes back 12 months from the selected date in a slicer.
Using this video -

I have this code:

TRI P12M = 
VAR ReferenceDate = MAX ( 'Date'[Date] )
VAR PreviousDates = 
    DATESINPERIOD (
        'Previous Date'[Date],
        ReferenceDate,
        -12,
        MONTH
    )
VAR Result =
    CALCULATE(
        MAX(
        'DataFreqRates'[TRI Month]),
        PreviousDates,
        USERELATIONSHIP ( 'Date'[Date], 'Previous Date'[Date])
    )
RETURN
     Result

I get what it is trying to do, but I am getting an error - MdxScript(Model) (18, 5) Calculation error in measure 'DataFreqRates'[TRI P12M]: an invalid numeric representation of a date value was encountered.
and looking it up it looks like I am trying to sum a date, but I don't know how or why.

Can someone guide me with what I am doing wrong, and how to fix?

More Info -
TRI Month is basically a performance metric for the end of the month.
DataFreqRates is the table that calculates several scores.

 

[tazeo]

-BUMP-
Any thoughts from anyone? I'm getting frustrated. I can't work it out.