Db2 – How to ensure all records of a header are under the same fiscal year

The easiest way is to have a calendar table, defined in the following way:

CREATE TABLE calendar
(
    a_day date PRIMARY KEY
) ;

... and filled with all the relevant dates (i.e.: all days from 1990-1-1 to 2100-12-31). For the sake of simplicity, we will fill it only with year 2013:

INSERT INTO 
     calendar (a_day)
VALUES
    ('2013-01-01'),
    ('2013-01-02'),
    ('2013-01-03'),
    ('2013-01-04'),
    ('2013-01-05'),
    -- everything up to
    ('2013-12-31') ;

At this point, you can just have a JOIN with the two tables; with the join condition not being an equality, but a range condition:

SELECT
     t.id, c.a_day
FROM
     t
     JOIN calendar c ON c.a_day BETWEEN t.start_date AND t.end_date 
ORDER BY
     t.id, c.a_day ;

... and get

id | a_day     
-: | :---------
 1 | 2013-01-14
 1 | 2013-01-15
 1 | 2013-01-16
 1 | 2013-01-17
 1 | 2013-01-18
 2 | 2013-02-01
 2 | 2013-02-02
 2 | 2013-02-03
 2 | 2013-02-04

You can see all the setup at dbfiddle here

You can use a hierarchical query to generate a sequence of dates, something like this:

insert into my_table (my_date_column) 
select dt + (level - 1) 
from (select date '<your start date>' dt from dual)
connect by dt + (level - 1) <= date '<your end date plus 1 day>';

Date arithmetics: dt + (level - 1) -- are smart enough to know when one month ends and another begins (including Februaries in leap years).

If you do need a PL/SQL function for this, I'm sure you will be able to wrap it around this statement easily.