INTERVAL FUNCTION

Hello, I am unsure of why this function behaves like it does (or more accurately, what the sql means)? create_date is a DATETIME YEAR TO SECOND select * from request wherecreate_date <= EXTEND(current, year to second) - interval (99) second to second why does the above statement work but when i replace 99 with 100, it fails? as i increase the number of seconds in the interval, i am required to change the sql to this: select * from request where create_date <= EXTEND(current, year to second) - interval (18000)second (5) to second what does the (5) mean and why do i have to increase it as i increase the interval seconds? the above sql will fail if it is a (4) instead of a (5). Thanks in advance, Tom