Code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ArriveOnTime
{
    class ArriveOnTime
    {
        static void Main(string[] args)
        {
            var examHour = int.Parse(Console.ReadLine());
            var examMinutes = int.Parse(Console.ReadLine());
            var hourArrive = int.Parse(Console.ReadLine());
            var minArrive = int.Parse(Console.ReadLine());

            string late = "Late";
            string onTime = "On time";
            string early = "Early";

            int examTime = (60 * examHour) + examMinutes;
            int arriveTime = (60 * hourArrive) + minArrive;

            int totalMinutesArrival = arriveTime - examTime;

            string studentArrival = string.Empty;
            if (totalMinutesArrival < -30)
            {
                studentArrival = early;
            }
            else if (totalMinutesArrival <= 30)
            {
                studentArrival = onTime;
            }
            else
            {
                studentArrival = late;
            }

            string result = string.Empty;
            if (totalMinutesArrival != 0)
            {
                int hoursDifference = Math.Abs(totalMinutesArrival / 60);
                int minutesDifference = Math.Abs(totalMinutesArrival % 60);

                if (hoursDifference > 0)
                {
                    result = string.Format("{0}:{1:00} hours", hoursDifference, minutesDifference);
                }
                else
                {
                    result = minutesDifference + " minutes";
                }
                if (totalMinutesArrival < 0)
                {
                    result += " before the start";
                }
                else
                {
                    result += " after the start";
                }
            }

            Console.WriteLine(studentArrival);
            if (!string.IsNullOrEmpty(result))
            {
                Console.WriteLine(result);
            }
        }
    }
}