SQL & PL/SQL :: To Make Report Of Processing Time For Tasks

I want to make report of processing time for tasks. Every task has several stages and connected with them modification dates. I need calculate total processing time for every task.

In my calculations i have to consider only "working" hours. There is time window table (For example working hours: from 8 a.m to 4 p.m, but can be different thats why i use table with id - different id = different time window). Scripts i used to simulate problem:

create table t (
id number,
s_date date,
e_date date
);
declare
[code]........

table t constrains "working" hours:

12011-04-10 08:00:002011-04-10 16:00:00
12011-04-11 08:00:002011-04-11 16:00:00
...
but could be also

22011-04-10 09:00:002011-04-10 17:00:00
22011-04-11 09:00:002011-04-11 17:00:00
...

table w contains tasks history processing by team (8 hours for tenet, rest time (out of hours defined in table t) of working trough incident should be ignored/omitted in calculations):

12011-04-10 10:00:00
12011-04-11 02:08:16
12011-04-11 16:23:10
22011-04-12 08:12:30
22011-04-13 00:40:15
32011-04-14 09:37:54
32011-04-14 15:54:36
32011-04-15 06:40:44
32011-04-16 09:48:06
32011-04-16 20:06:01
32011-04-18 14:11:29

At the beginning I tried this query, but I don't know how consider/dose time window from table t

select id, change_date,
round( to_number(change_date-lag(change_date) over (partition by id order by change_date))*24*60*60) sec
from w;

12011-04-10 10:00:00
12011-04-11 02:08:1658096 - sholud be 21600 (sum time only for 2011-04-10 16:00:00)
12011-04-11 16:23:1051294 - should be 28800 (sum time only from 2011-04-11 08:00:00 to 2011-04-11 16:00:00)
22011-04-12 08:12:30
22011-04-13 00:40:1559265 - should be 28050 (sum time only from 2011-04-12 08:12:30 to 2011-04-12 16:00:00)
32011-04-14 09:37:54