Postgres 12

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
create table mytable (Time date, sum_quantity int)

CREATE TABLE

insert into mytable values('01-02-2020', 1)

INSERT 0 1

insert into mytable values('02-02-2020', 2)

INSERT 0 1

insert into mytable values('03-02-2020', 3)

INSERT 0 1

insert into mytable values('04-02-2020', 9)

INSERT 0 1

insert into mytable values('01-01-2020', 9)

INSERT 0 1

select sum(sum_quantity)
from mytable
where time between (time - interval '29 day') and (time) 

sum

24

SELECT 1

    select m1.Time, (select sum(sum_quantity) 
                     from mytable m
                     where m.time between (m1.time - interval '29 day') and (m1.time)) sum_total
    from mytable m1
    group by m1.Time
    order by m1.Time;

time
sum_total

2020-01-01
9

2020-02-01
1

2020-02-02
3

2020-02-03
6

2020-02-04
15

SELECT 5

    select m1.Time, sum(m.sum_quantity) 
    from mytable m
    join mytable m1 on m.time between (m1.time - interval '29 day') and (m1.time)
    group by m1.Time
    order by m1.Time;

time
sum

2020-01-01
9

2020-02-01
1

2020-02-02
3

2020-02-03
6

2020-02-04
15

SELECT 5