The following query:
SELECT ROUND(CAST('2003/09/15' AS DATE), 'MONTH') (FORMAT 'yyyy-mm-dd');
returns the result 2003-09-01. Since the day is less than 16, the date returns to the beginning of the beginning of the month.
The following query:
SELECT ROUND(CAST('2003/09/17' AS DATE), 'RM') (FORMAT 'yyyy-mm-dd');
returns the result 2003-10-01. Since the day is greater than or equal to 16, the date is rounded to the beginning of the next month.