It seems in your example you're counting events from last month? I'm not sure what Now(-1)
is trying to calculate in your example.
Anyway, this example worked for me in identifying counts from last month:
=Sum(IIf(DatePart("m", Fields!RequestDate.Value) = DatePart("m", DateAdd("m", -1, Now()))
, 1
, 0))
You can update the DateAdd("m", -1, Now())
part of the expression to set the month you want to update against.