Function To Find Business Days Between Two Dates - Query Tuning

I have a query that uses a function to find the business days between two dates.It sums the total number of days between two dates per employee to find the total days for the past 30, 90, or 365 days.

The problem is that the query takes 21 second to return the last 30 days.Over 70 second to return the last 90 days and over 140 second to return the last 365 days.Do you know how I could tune the query to return faster? Below is the query for the last 30 days:

select dwt_emp_id, SUM((SELECT GET_BDAYS(DWT_DATE,DWT_CREATE_DATE) FROM DUAL))
from dwt_dvt_work_time where dwt_create_date > sysdate - 30
and dwt_hours > 4 and dwt_usr_uid_created_by <> -1 group by dwt_emp_id order by dwt_emp_id

Here's the function:
CREATE FUNCTION get_bdays (d1 IN DATE, d2 IN DATE)
RETURN NUMBER
IS total_days NUMBER(11,2);
holiday_days NUMBER(11,2);
[code]....