Check that the ASCII string that contains a DATETIME or INTERVAL value conforms to the format string. For example, a percent character in a DATETIME or INTERVAL ASCII string must have a matching "%%" sequence in the format string. See also the discussion of the DBTIME environment variable in the IBM Informix Guide to SQL: Reference.
If you are using the ROUND() or TRUNC() build-in functions on DATE or DATETIME data types, note that the list of supported values for the second parameter is "YEAR", "MONTH", "DAY", "DD", "HH" or "MI". Please refer to the discussion on "Algebraic Functions" in the IBM Informix Guide to SQL: Syntax for the details.