Fixed code +15 minutes

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

namespace _1_TEST
{
    class Program
    {
        static void Main(string[] args)
        {

            var hour = int.Parse(Console.ReadLine());
            var min = int.Parse(Console.ReadLine());
            min = min + 15;

            if (min >= 60)
            {
                min = min - 60;
                hour = hour + 1;

            }

            if (hour > 23)
                { hour = hour - 24; }
            if (min < 10)
            {
                Console.WriteLine(hour + ":" + "0" + min);
            }
            if (min >= 10)
            {
                Console.WriteLine(hour + ":" + min);
            }
        }
    }
}