Oracle 18c

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create table btest
(
  name varchar(100),
  bday date
);
insert into btest values ('Zaphod', to_date('1990'||to_char(sysdate - 10, 'mmdd'), 'yyyymmdd'));
1 rows affected

insert into btest values ('Arthur', to_date('1990'||to_char(sysdate, 'mmdd'), 'yyyymmdd'));
1 rows affected

insert into btest values ('Trica', to_date('1990'||to_char(sysdate + 5, 'mmdd'), 'yyyymmdd'));
1 rows affected

insert into btest values ('Ford', to_date('1990'||to_char(sysdate + 60, 'mmdd'), 'yyyymmdd'));
1 rows affected

with bdays as (
  select name, bday, 
         case 
           when to_char(sysdate, 'mmdd') <= to_char(bday, 'mmdd') 
              then to_date(  to_char(extract(year from sysdate), 'FM0000')||to_char(bday, 'mmdd'), 'yyyymmdd')
           else
            to_date(  to_char(extract(year from sysdate) + 1, 'FM0000')||to_char(bday, 'mmdd'), 'yyyymmdd')
         end as next_bday
  from btest
)
select *
from bdays
--where next_bday <= sysdate + 30


NAME
BDAY
NEXT_BDAY


Zaphod
04-APR-90
04-APR-21

Arthur
14-APR-90
14-APR-20

Trica
19-APR-90
19-APR-20

Ford
13-JUN-90
13-JUN-20

NAME	BDAY	NEXT_BDAY
Zaphod	04-APR-90	04-APR-21
Arthur	14-APR-90	14-APR-20
Trica	19-APR-90	19-APR-20
Ford	13-JUN-90	13-JUN-20