Data Model Default parameter to current period

On a Data Model for a report I have a parameter for the period that I would like to default to the current period. The parameter is a Data Type of Date and a Parameter Type of Date. When I try setting the default value to {TO_CHAR(SYSDATE,'Mon-YY','NLS_DATE_LANGUAGE=AMERICAN')} I receive an error of Invalid Date Format. When I use {$SYSDATE()$} it does default to the current date but I need the current period