Unlike T-SQL, there is no BETWEEN keyword in SSRS expressions, at least that I am aware of. But, you can always use the And operator to specify a date range - for example, this would pick the 24 months ago to 12 months ago:=iif(DateDiff("M",Fields!TIME_DATE.Value,Now()) > 12 And DateDiff("M",Fields!TIME_DATE.Value,Now()) <= 24,Fields!AMOUNT.Value,NOTHING)
One thing to keep in mind is that when you do DATEDIFF("M" it is counting the number of month boundaries crossed. So DateDiff("M","1/31/2012","2/1/2012") (Jan 31 to Feb 1) would return 1, as would DateDiff("M","1/31/2012","2/29/2012")