updated code, program still does not work, i'm pulling my hair out now ahhhhh, enough for the day.

Code:
#include "stdafx.h"
#include <iostream>
#include <string>
#include <iomanip> // setprecision() funcion
#include <conio.h> // _getch() function
#include <ctime> // add system time and date
using namespace std;

const float interest = 1.67;
const int cust_account_number = 112280;

string first_name, customer_surname;
double balance, deposit, withdrawl;
int account_exists = 0;
int account_number;
char dat[10], tim[9];

void display_main_menu();
void open_account();
void account_withdrawl();
void account_deposit();
void add_interest();
void display_account_details();
void exit();

void display_main_menu()
{
	int choice;
	do
	{
			system ("cls");
			_strdate(dat);
			_strtime(tim); 

			cout << endl << "Todays date is: " << dat << " and the time is " << tim << endl << endl;
			cout << "\t  Bank Account Menu" << endl;
			cout << "\t_________________________" <<endl << endl;
			cout << "\t1.  Open an account" << endl
				<<"\t2.  Make a withdrawl" << endl
				<<"\t3. Make a deposit" << endl
				<<"\t4. Add interest" << endl
				<<"\t5.  Exit" << endl << endl
				<<"Enter Choice: ";
			
			cin >> choice;
			
			system ("cls");

	switch(choice)
	{
	case 1: 
		open_account();
	break;
	case 2: 
		account_withdrawl();
	break;
	case 3: 
		account_deposit();
	break;
	case 4: 
		add_interest();
	break;
	case 5: 
		exit();
		cout << "\t Exit main menu" << endl << endl;
	break;

	}// End of switch statement
	
	} // End of loop
while(choice !=5);

} //End of 'display_main_menu' method

void open_account()
// allocate account number, get initial balance and confirmation message that account has been
// created
{	
	cout << "Enter customer first name: ";
	cin >> customer_first_name;
	cout << "Enter customer surname: ";
	cin >> customer_surname;
	account_number = account_number + 1;
	cout << endl << "Enter initial balance: ";
	cin >> balance;
	cout << endl << "Congratulations on opening your new bank account with us" << endl << endl;
	account_exists = 1;
	
	display_account_details();

	cout << endl << endl << "Please press any key to return to the main menu" << endl << endl;
	_getch();
}

void account_withdrawl()
{
// if account exists prompt user for withdrawal amount if sufficient funds to withdraw cash
// or else print account does not exist message
if (account_exists == 1)
{
	cout << endl << "Enter how much you wish to withdraw: ";
	cin >> withdrawl;
	if (withdrawl > balance)
	cout << "You have insufficient funds in your bank account to cover this withdrawl!" 
	<< endl;
	else
		{
		balance = balance - withdrawl;
		cout << endl << (char)156 << withdrawl << " has been withdrawn from your account. " << endl;
		display_account_details();
		}
	} //End of first 'if' statement.
	else
	
	cout << "Account does not exist!" << endl << "Press any key to return to the main menu. " << endl;
	_getch(); 
}

void account_deposit()
{
/*if account exists prompt for deposit amount, add deposit to balance
else print account does not exist message
end if
*/

if (account_exists == 1)
{
	cout << "Please enter the amount you wish to deposit: ";
	cin >> deposit;

	balance = balance + deposit;

	display_account_details();
}// End of True "if" statement

else
	cout << "Account does not exist!" <<endl << "Press any key to return to the main menu." << endl;
	_getch();

}

void add_interest()
{
	float interest_added = 0;

	if (account_exists == 1)
		{
			interest_added = balance * (interest/100);
			cout << (char) 156 << fixed << setprecision(2) <<
interest_added << "has been added to the following account." << endl << endl;
			balance = balance + interest_added;
			display_account_details();
		} // End of true 'if' statements
else
		cout << "Account does not exist!" << endl << "Press any key to return to the main menu. " << endl;
		_getch();
}

void display_account_details()
{
	cout << "\t Account Details" << endl << endl;
	cout << "\t Customer Name:  " << customer_first_name << " " <<
customer_surname << endl;
	cout << "\t Account Number: " << "00" << account_number << endl;
	cout << "\t Current Balance: " << (char)156 << fixed << setprecision(2)
		<< balance << endl; //(char)156 inserts the symbol ''.

}

int main()
{
display_main_menu();
}