You could try an expression like this. (Assuming you expression is Sum(Amount),
sum(aggr(rangesum(above(total sum({<YearMonth=,Month=,Year=,Date= /*All date fields must be ignored/listed here*/>} Amount),0,12)),YearMonth))
Another approach is to create an as-of table. Take a look at HICs old blog post: "The As-of-table". It is a quite good approach for enabling R12 calculations.
