Help with UDFs or stored procedures

(Cross-posted to midrange-l and rpg400-l)

Our legacy database files are littered with dates defined as four numeric
fields, with a 2-digit century, 2-digit year, month, and day.  I must work
with these dates in SQL and WrkQry, sometimes in RPG.  I would like to
define either a UDF or stored procedure or *something* that would convert
four input parameters into a date result.  It would be nice if I could use
the same function in an SQL statement and in RPG code, but not a
requirement.

So, I need to read up on how to do this, and how to choose UDF vs. stored
procedure.  I searched the archives, saw a few examples of code, but did not
see any reference on how to call the UDF or stored procedure.

Can anyone suggest good online references?

TIA,
Dan