#include <iostream>
#include <cstdlib>
#include <ctime>   //for time_t, time, tm, localtime
using namespace std;

void travel(int& month, int& day, int& year, int distance);

int main()
{

	//Get the current year, month, and day of the month.
	time_t t {time(nullptr)};
	tm *p {localtime(&t)};

	int y {p->tm_year + 1900};
	int m {p->tm_mon + 1};   //in the range 1 to 12 inclusive
	int d {p->tm_mday};      //in the range 1 to 31 inclusive

	cout << "How many days forward do you want to go from "
		<< m << "/" << d << "/" << y << "? ";
	int distance {0};
	cin >> distance;

	if (!cin) {
		cerr << "Sorry, that wasn't a number.\n";
		return EXIT_FAILURE;
	}

	if (distance < 0) {
		cerr << "Distance " << distance << " can't be negative.\n";
		return EXIT_FAILURE;
	}

	//The following statement can change the value of m, d, y,
	//but not the value of distance.
	travel(m, d, y, distance);

	cout << "The new date is "
		<< m << "/" << d << "/" << y << ".\n";

	return EXIT_SUCCESS;
}

//Move the date distance days into the future.

void travel(int& month, int& day, int& year, int distance)
{
	static int length[] { //no need to recreate this array with every call
		 0,   //dummy, so that January will have subscript 1
		31,   //January
		28,   //February, assuming there are no leap years
		31,   //March
		30,   //April
		31,   //May
		30,   //June
		31,   //July
		31,   //August
		30,   //September
		31,   //October
		30,   //November
		31    //December
	};

	static int n {size(length) - 1}; //#of months in year,not counting dummy

	//Walk distance days into the future.
	//Each iteration of the loop advances 1 day into the future.
	//Each ++ will now change the value of an actual argument up in main.

	for (int i {0}; i < distance; ++i) {
		if (day < length[month]) {
			++day; 
		} else {
			day = 1;             //Advance into a new month.
			if (month < n) {
				++month;
			} else {
				month = 1;   //Advance into a new year.
				++year; 
			}
		}
	}
}