What is result of SQL DATE() scalar function when argument is not a valid date?

I'm trying for an SQL statement that will identify invalid dates in a file that stores dates as 3 separate fields, year, month and day. Something like

SELECT DCTR#,DBRDTY,DBRDTM,DBRDTD, case when date(digits(dbrdty) || '-' ||
digits(dbrdtm) || '-' ||
digits(dbrdtd)) is null
then 'ER' else 'OK' end FROM QTEMP/T_GPPDCTR

Using the above statement, the invalid dates are displayed as '++', the valid dates as 'OK'.

If I try

SELECT DCTR#,DBRDTY,DBRDTM,DBRDTD, case when date(digits(dbrdty) || '-' ||
digits(dbrdtm) || '-' ||
digits(dbrdtd)) = '++'
then 'ER' else 'OK' end FROM QTEMP/T_GPPDCTR

I get "Comparison operator = operands not compatible.". The manual is less than forthcoming about what is returned when the argument is not a valid date.

*Peter Dow* /
Dow Software Services, Inc.
909 793-9050
pdow@xxxxxxxxxxxxxxx <mailto:pdow@xxxxxxxxxxxxxxx> /