using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        var time = "130411";

        var result = ApplySecondsToHourString(time, 20);

​

        Console.WriteLine(result);

    }

​

    public static string ApplySecondsToHourString(string hour, int seconds)

    {

        DateTime dateTime = DateTime.ParseExact(hour, "HHmmss",

                                        CultureInfo.InvariantCulture);

​

        dateTime = dateTime.AddSeconds(seconds);

        return dateTime.ToString("HHmmss");

    }

}