I find that the easiest way to manage Transformation Rules which need to vary by period is to bring the period into the stage data. Select a dimension that currently maps all records to a single member (e.g. Mask * to None). This means that anything you import into this dimension will not change the Cube data. Add a Logical Operator | Business Rule to that dimension on the Data Source that brings in the current period. And you can then use that in the Derivative rule. If you want to exclude a particular period you might want to bring in something like M1-Yes, M2-Yes, M3-No etc so that you only have one record in the Derivative Rule.
