Calculating Time Difference in military time for C++ - c++

Question

I have to write a program that gives the difference between two times that are given in military time format. For example,
Please enter the first time: 1730
Please enter the second time: 0900
Output = 15 hours 30 minutes
Here is the program I came up with (*Not allowed to use if statements, loops or functions)
I just need to know whether this is correct or incorrect?
int main()
{
int first;
int second;
cout << "Please enter the first time:";
cin >> first;
cout << endl;
cout << "Please enter the second time:";
cin >> second;
cout << endl;
int am = first - 1200;
second = second + 1200;
int amhour = am/100;
int ammins = am%100;
int hours = second - amhour*100;
int real_hr = hours - 40;
int final_time = real_hr - ammins;
int final_hours = final_time/100;
int final_mins = final_time%100;
cout << final_hours << " hours and " << final_mins << " minutes." << endl;
return 0;
}

Answer 1

I would first convert each time into "minutes since midnight", then subtract the first from the second, then use that to form your output.
However, you may need to take into account that the subtraction may give you a negative number if, for example, the first time is 2330 and the second is 0030 (crossing the day boundary).
You can fix that by simply adding 1440 minutes (one day) to the difference and then using the modulo operator to reduce that if it's more than a day.
So I would start with something like:
#include <iostream>
int main() {
// Get times from user.
int first, second;
std::cout << "Please enter the first time: "; std::cin >> first;
std::cout << "Please enter the second time: "; std::cin >> second;
// Convert both to minutes since midnight.
first = (first / 100) * 60 + first % 100;
second = (second / 100) * 60 + second % 100;
// Work out time difference in minutes, taking into
// account possibility that second time may be earlier.
// In that case, it's treated as the following day.
const int MINS_PER_DAY = 1440;
int minutes = (second + MINS_PER_DAY - first) % MINS_PER_DAY;
// Turn minutes into hours and residual minutes, then output.
int hours = minutes / 60;
minutes = minutes % 60;
std::cout << hours << " hours and " << minutes << " minutes.\n";
}