Calculate Working Hours between two dates in Postgres

fn.sql

create or replace
function calculate_working_hours(start_time TIMESTAMPTZ,
end_time TIMESTAMPTZ)
returns interval as $$
declare
    date record;
  minutes interval := interval '0 minutes';
begin
for date in
select
	*
from
	generate_series(start_time,
	end_time,
	'1 minutes')
where
	generate_series::time between interval '09:00:00' and interval '18:00:00'
	and extract('dow'
from
	generate_series) not in (0, 6)
 loop

minutes := minutes + interval '1 minutes';
end loop;

return minutes;
end;

$$ language plpgsql