Customer Balance - receivable

Dear All,

how can i calculate the balance till a specific date ex: till 01-JAN-2018 of a customer

im using these functions but it still missing something because the amount isnt correct:

"

function cust_open_balFormula return Number is

  open_bal_INV number;

  open_bal_payment number;

  open_bal_adj number;

  net_bal  number;

total_refund_bf number;

begin

  select nvl(sum(rcpt.functional_amount),0)

  into   open_bal_payment

from

apps.ar_cash_receipts_v rcpt

where

rcpt.customer_number = :cust_num AND

rcpt.gl_date < nvl(:P_TRX_FROM_DATE,to_date('01-JAN-2006','DD-MON-YYYY')) AND

rcpt.gl_date > '31-DEC-05' AND