+15 ми

            var hour = int.Parse (Console.ReadLine ());
            var min = int.Parse (Console.ReadLine ())+15;
            if (min > 59) {
                hour = hour + 1;
                min = min - 60;
            }
            if (hour > 23)
                hour = hour - 24;
            if (min<10)
                Console.WriteLine (hour+":0"+min);
            else
                Console.WriteLine (hour+":"+min);