create table maint_sched (maint_id number, start_date date, day_interval number);
insert into maint_sched values (1, date '2015-01-01', 90);
insert into maint_sched values (2, date '2015-04-01', 200);
insert into maint_sched values (3, date '2015-07-01', 30);

MAINT_ID   DATE_DUE
--------  ---------
1          1-Jan-15
1          1-Apr-15
2          1-Apr-15
1         30-Jun-15
3          1-Jul-15
3         31-Jul-15
3         30-Aug-15
1         28-Sep-15
3         29-Sep-15
2         18-Oct-15
3         29-Oct-15
3         28-Nov-15
1         27-Dec-15
3         28-Dec-15

-- This generates a list of every five days for 36 months
SELECT (sysdate + (5 * level)) AS duedate
FROM dual
CONNECT BY (sysdate + (5 * level)) < add_months(sysdate, 36)