Rolling total for last month as caluclated column

[dicken]

Hi
I've been trying o write a Rolling total for last month as calculated column not a measure,
but I've tried using EDATE as I would in excel and DATESADD, but these do not seem compatible with EARLIER, and without just return a running total,
so just getting a total last 3 days; how do I get it to look back over the last month?

Richard.

CALCULATE (
    SUMX ( fSales, fSales[Sales] ),
    FILTER (
        ALL ( fSales ),
        fSales[Order Date] <= EARLIER ( fSales[Order Date] )
            && fSales[Order Date]
                > EARLIER ( fSales[Order Date] ) - 3
    )
)