I don't see the difference between your example and what I've done - well, yes, you have a formula for the month in the group itself whereas I have the formula in the script already. But it doesn't work yet. Have I overlooked something?
I can create a drilldown group, but then I'm stuck with the dimension >month<, the >day< never appears.
When I change it into a cyclic_group, it works fine.
I have a date in the original base data. In the script, I have created a month_field. That's all there is.
I've also just tried doing that calculation (IF...>=Monthstart(TODAY()) in the script, but that doesn't change things.