calculating user time and time elapsed

Hi im a little bit stuck on this program im working on.

im creating a program which uses the data type Mytime the output should look something like

Enter start hh mm : 7 30

Enter finish hh mm : 7 30

Time elapsed from 07:30 until 07:30 is 00:00

2 functions have not yet been defined display1Time() & elapsed(). The program currently compiles in unix g++ i have filled the un defined function to get the program running. Basically i cant work out how to get the time displayed and the total time elapsed working. I think the time the user enters has to be converted to minutes. Can anyone help please ?

Code:

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// Incomplete version
#include <iostream>
using namespace std;

struct Mytime
{
  int hours;
  int mins;
};

int timecmp(Mytime,Mytime);
// returns 0 if times are same
// returns >0 if first is later than second
// returns <0 if first is before second

int setTime(Mytime& t, int h, int m );
// if valid time sets hh and mm to h and m
// and returns 0
// if invalid returns integer > 0 as error code
// error code +1 = underflow hours
// error code +2 = overflow hours
// error code +4 = underflow mins
// error code +8 = overflow mins

void display1Time(Mytime t);
// displays in form hh:mm

void elapsed(Mytime t1, Mytime t2, Mytime& duration);
// determines the time duration between t1 and t2
// t1 assumed to be earlier than t2

void main()
{
    Mytime now;
    Mytime then;
    Mytime howlong;
   
    int h,m;
   
    do // validate input
    {
        cout << "Enter start hh mm : ";
        cin >> h >> m ;
    } while (setTime(now,h,m));
   
    do
    {
        cout << "Enter finish hh mm : ";
        cin >> h >> m ;
    } while (setTime(then,h,m));
   
    elapsed(now,then,howlong);

    cout << "Time elapsed from ";
    display1Time(now);
    cout << " until ";
    display1Time(then);
   
    cout << " is ";
    display1Time(howlong);
    cout << endl;
}

int timecmp(Mytime t1,Mytime t2)
{
  if (t1.hours == t2.hours)
  {
      if (t1.mins == t2.mins)
      {
        return 0;
      }
      else // mins not same
      {
        if (t1.mins > t2.mins)
        {
            return 1; // greater than zero
        }
        else
        {
            return -1;
        }
      }
  }
  else // hours not same
  {
      if (t1.mins > t2.mins)
      {
        return 1;
      }
      else
      {
        return -1;
      }
  }
}

int setTime(Mytime& t, int h, int m )
{
    int errorCode = 0;
    if (h < 0) errorCode += 1;
    if (h > 23) errorCode += 2;
    if (m < 0) errorCode += 4;
    if (m > 59) errorCode += 8;
    if (errorCode==0){t.hours = h; t.mins = m;}
    return errorCode;
}

//////////// THE FOLLOWING FUNCTIONS ARE INCOMPLETE

void display1Time(Mytime t)
{
    cout << "00:00";
}


void elapsed(Mytime t1, Mytime t2, Mytime& duration)
{
    setTime(duration,0,0);
}

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