How get start and end date from date colum

You can mimic the effect of LEAD or LAG in earlier versions of DB2 by using a self-join and a row number. This is a much simplified demo in T-SQL (I no longer have access to DB2, sorry)

-- demo table with dummy data
declare @demo table (id int identity(1,1), cdata varchar(100));
insert into @demo (cdata) values
('A'),('B'),('C'),('D'),('E'),('F'),('G'),('H');

-- equivalent of LAG
select *
from @demo a
left outer join @demo b on b.id = a.id -1;

gives results

id	cdata	id	cdata
1	A		NULL	NULL
2	B		1		A
3	C		2		B
4	D		3		C
5	E		4		D
6	F		5		E
7	G		6		F
8	H		7		G

Equivalent of LEAD

select *
from @demo a
left outer join @demo b on b.id - 1 = a.id;

which gives results

id	cdata	id	cdata
1	A		2		B
2	B		3		C
3	C		4		D
4	D		5		E
5	E		6		F
6	F		7		G
7	G		8		H
8	H		NULL	NULL

Note - you should not rely on an ID column - the values will be unique but not necessarily contiguous, meaning the join will not work. Use instead a row number[^] in a sub-query, common table expression or temporary table - use the latter with an ID column if your version of DB2 does not support row_number()