[Help very much appreciated] with time functions and calculations

[AwkwardRaven]

I'm writing a program to calculate a fee based on the time that a car is parked in a theoretical garage. But my time function doesn't seem to be giving me the correct number of minutes... I would like to use the difftime function but I'm not sure exactly how to input my integers into the correct syntax in order to utilize said function.

Here's what I have so far:

//Created by John Pearce
//
//Version 1.54
#include <iostream>
#include <string>
#include <iomanip>
#include <cmath>
#include <cstring>
using namespace std;

int feeTime = 0;
float fee = 0;
int enteringMins = 0, exitingMins = 0;
int enterY, enterM, enterD, enterH, enterMi, exitY, exitM, exitD, exitH, exitMi;
string entering, exiting;

void convertStringToInt();
void calculateParkingHours();
float calculateFee(int);

int main ()
{
	cout << "Welcome to John's Parking Garage fee calculator." << endl;
	cout << "fees are as follows: $2.00 minimum up to three hours." << '\n'
		<< "An additional $0.50 per hour." << '\n'
		<< "$8.00 Flat fee for every 24 hours." << endl;
	cout << setw(70) << setfill('*') << '\n' << endl;

		cout << "Please enter the date and time the car is entering the parking garage" << '\n'
			<<  "in the following format: MM/DD/YY hh:mm" << endl;
		getline(cin, entering);
		cout << '\n' << "Please enter the date and time the car is exiting the parking garage" << '\n'
			<< "in the following format: MM/DD/YY hh:mm" << endl;
		getline (cin, exiting);
		
		convertStringToInt();
		calculateParkingHours ();
		calculateFee(feeTime);
		
		cout << '\n' << "Number of min parked: " << feeTime << ".  Fee: $" << setprecision(2) << fixed << fee << endl;

return 0;
}

void convertStringToInt()
{
	enterY = atoi ( entering.substr( 6, 2 ).c_str() );
	enterM = atoi ( entering.substr( 0, 2 ).c_str() );
	enterD = atoi ( entering.substr( 3, 2 ).c_str() );
	enterH = atoi ( entering.substr( 9, 2 ).c_str() );
	enterMi = atoi ( entering.substr( 12, 2 ).c_str() );
	
	exitY = atoi ( exiting.substr( 6, 2 ).c_str() );
	exitM = atoi ( exiting.substr( 0, 2 ).c_str() );
	exitD = atoi ( exiting.substr( 3, 2 ).c_str() );
	exitH = atoi ( exiting.substr( 9, 2 ).c_str() );
	exitMi = atoi ( exiting.substr( 12, 2 ).c_str() );
}

void calculateParkingHours ()
{
enteringMins += ( enterY * 365 * 29 * 60 );
enteringMins += ( enterM * 12 * 29 * 60 );
enteringMins += ( enterD * 29 * 60 );
enteringMins += ( enterH * 60 );
enteringMins += ( enterMi );

exitingMins += ( exitY * 365 * 29 * 60 );
exitingMins += ( exitM * 12 * 29 * 60 );
exitingMins += ( exitD * 29 * 60 );
exitingMins += ( exitH * 60 );
exitingMins += ( exitMi );

feeTime = ((enteringMins - exitingMins) * -1);
}

float calculateFee(int feeTime)
{
	if (feeTime >= 1440)
	{
		fee = 8.00;
		if (feeTime >= 2880)
		{
			fee += ((feeTime - 1440) / 1440) * 8.00;
			if ((feeTime - 1440) % 1440)
			fee += 8.00;
		}	
	}
	else if (feeTime >= 180)
	{
		fee = 2.00;
		if (feeTime > 180)
		{
			fee += ((feeTime - 180) / 60) * 0.5;
			if ((feeTime - 180) % 60)
			fee += .50;
		}
		if (fee > 10)
			fee = 10;
	}
	else if (feeTime < 180)
	{
		fee = 2.00;
	}

	return fee;
}