using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        //double test = 12686879.08497765;

        //string str = Convert.ToString(TimeSpan.FromSeconds(test));

        //TimeSpan interval = new TimeSpan();

        double timeToCutOff = 1278989912.08497765;        

        var ts = TimeSpan.FromSeconds(timeToCutOff);        

        string formattedTimeSpan = string.Format("{0}:{1:D2}:{2:D2}", ts.Days * 24 + ts.Hours, ts.Minutes, ts.Seconds);

        Console.WriteLine(formattedTimeSpan);

        Console.WriteLine("-----------------------------------------------");

         var totalSeconds = Math.Floor(ts.TotalSeconds);

         var hours = Math.Floor(totalSeconds / 3600).ToString().PadLeft(2, '0');

         var minutes = Math.Floor((totalSeconds % 3600) / 60).ToString().PadLeft(2, '0');

         var seconds= Math.Floor(totalSeconds % 60).ToString().PadLeft(2, '0');

         var formattedInterval = $"{hours}:{minutes}:{seconds}";

        Console.WriteLine(formattedInterval);

    }

}

​