How can I convert seconds into (Hour:Minutes:Seconds:Milliseconds) time?

For .Net <= 4.0 Use the TimeSpan class.

TimeSpan t = TimeSpan.FromSeconds( secs );

string answer = string.Format("{0:D2}h:{1:D2}m:{2:D2}s:{3:D3}ms", 
                t.Hours, 
                t.Minutes, 
                t.Seconds, 
                t.Milliseconds);

(As noted by Inder Kumar Rathore) For .NET > 4.0 you can use

TimeSpan time = TimeSpan.FromSeconds(seconds);

//here backslash is must to tell that colon is
//not the part of format, it just a character that we want in output
string str = time .ToString(@"hh\:mm\:ss\:fff");

(From Nick Molyneux) Ensure that seconds is less than TimeSpan.MaxValue.TotalSeconds to avoid an exception.

If you know you have a number of seconds, you can create a TimeSpan value by calling TimeSpan.FromSeconds:

 TimeSpan ts = TimeSpan.FromSeconds(80);

You can then obtain the number of days, hours, minutes, or seconds. Or use one of the ToString overloads to output it in whatever manner you like.

TimeSpan.FromSeconds(80);

http://msdn.microsoft.com/en-us/library/system.timespan.fromseconds.aspx

The TimeSpan constructor allows you to pass in seconds. Simply declare a variable of type TimeSpan amount of seconds. Ex:

TimeSpan span = new TimeSpan(0, 0, 500);
span.ToString();

In VB.NET, but it's the same in C#:

Dim x As New TimeSpan(0, 0, 80)
debug.print(x.ToString())
' Will print 00:01:20

I'd suggest you use the TimeSpan class for this.

public static void Main(string[] args)
{
    TimeSpan t = TimeSpan.FromSeconds(80);
    Console.WriteLine(t.ToString());

    t = TimeSpan.FromSeconds(868693412);
    Console.WriteLine(t.ToString());
}

Outputs:

00:01:20
10054.07:43:32

For .NET < 4.0 (e.x: Unity) you can write an extension method to have the TimeSpan.ToString(string format) behavior like .NET > 4.0

public static class TimeSpanExtensions
{
    public static string ToString(this TimeSpan time, string format)
    {
        DateTime dateTime = DateTime.Today.Add(time);
        return dateTime.ToString(format);
    }
}

And from anywhere in your code you can use it like:

var time = TimeSpan.FromSeconds(timeElapsed);

string formattedDate = time.ToString("hh:mm:ss:fff");

This way you can format any TimeSpanobject by simply calling ToString from anywhere of your code.

to get total seconds

var i = TimeSpan.FromTicks(startDate.Ticks).TotalSeconds;

and to get datetime from seconds

var thatDateTime = new DateTime().AddSeconds(i)

This will return in hh:mm:ss format

 public static string ConvertTime(long secs)
    {
        TimeSpan ts = TimeSpan.FromSeconds(secs);
        string displayTime = $"{ts.Hours}:{ts.Minutes}:{ts.Seconds}";
        return displayTime;
    }