My mistake was to grab Date.dim when I should have grabbed Due Date from the metadata tab. So the correct syntax is:

SELECT {Measures.[Reseller Sales Amount], Measures.[Reseller Sales Moving Average]} ON 0,

NONEMPTY([b][u]Due Date[/u][/b].[Calendar].[Month].Members) ON 1

FROM [Reseller Sales] -- reseller sales is the name of my cube