The following queries:
SELECT TRUNC(CAST('2003/09/15' AS DATE), 'MONTH') (FORMAT 'yyyy-mm-dd');
SELECT TRUNC(CAST('2003/09/30' AS DATE), 'MON') (FORMAT 'yyyy-mm-dd');
SELECT TRUNC(CAST('2003/09/17' AS DATE), 'MM') (FORMAT 'yyyy-mm-dd');
SELECT TRUNC(CAST('2003/09/17' AS DATE), 'RM') (FORMAT 'yyyy-mm-dd');
all return the result 2003-09-01. The date was truncated to the beginning of the month.