The function you need is CURRENTPERIODSTART() which takes the setting defined in Time.
Since CURRENTPERIODSTART() returns a date, you can convert this to a Period with the following formula PERIOD(CURRENTPERIODSTART()). If you really want to be slick, you can create a module defined by Time, call it Time Filters, and create a line item (Is Current Month?) with a format of Boolean with this formula: ITEM(Time) = PERIOD(CURRENTPERIODSTART()).
This gives you the ability to check if the month is defined as the current period month (if .Time Filters'.'Is Current Month?' then ...... else .....).