Filter time dim at run-time

[bdbits]

I have a cube with a time dim with granularity down to the day. I want to filter this to the last day of the previous month at runtime, i.e. [mydate] = _add_days ( _first_of_month ( current_date ), -1 ). I know how to build this as a detail filter expression, but I also know I should use a slicer, this being a cube. Normally you would drag a member in, so how do I use a slicer with an expression instead? I tried to think of a way to build a MUN at runtime but I really am clueless on how I would find the member to match the expression.

I just know I am going to say 'duh' when I have the solution in hand. Because my head hurts from banging it against the cubicle wall, even though they are padded, I thought I would embarrass myself by asking here. ;-)