SQL0181 and the IBM Date Window...

All,
 
The User Defined Function that follows is receiving an SQL0181 error..."
Value in date, time, or timestamp string not valid." when it is passed a
date that falls outside the IBM date window (1940-2039).  This is
confusing because a long julian date contains a full 4 digit year.  Why
would the date window even come into play on this?
 
If this is indeed a restriction of the DATE data type, does anyone have
any suggestions on how to convert a 7 digit long julian date (passed as
an integer) using SQL where we could put it into a function like this?
I didn't write the code below and I am not an SQL expert by any means,
so please go easy on me.
 
Best regards,
Dane
 
CREATE FUNCTION TEST/JDTODATE

 (JD INTEGER)

 RETURNS DATE

 LANGUAGE SQL

 SPECIFIC TEST/JDTODATE

 NOT DETERMINISTIC

 CONTAINS SQL

 CALLED ON NULL INPUT

 DISALLOW PARALLEL

 BEGIN

 DECLARE VAR1 DATE;

 IF JD = 0 THEN SET VAR1 = NULL;

 ELSE SET VAR1 = DATE(DAYS(CONCAT(CAST(INTEGER(JD)/1000 AS CHAR (4)),

  '-01-01'))+MOD(INTEGER(JD),1000) - 1);

 END IF;

 RETURN VAR1;

 END

 
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