Main.cs

using System;

class MainClass
{
    public static void Main (string[] args)
    {
        var dateTime1 = new DateTime(1970, 1, 1, 0, 0, 0);
        var dateTime2 = new DateTime(2020, 4, 1, 13, 30, 22, 500);

        TimeSpan span = dateTime2 - dateTime1;

        Console.WriteLine(span.ToString());
        // => 18353.13:30:22.5000000
        
        Console.WriteLine(span.ToString(@"hh\:mm\:ss"));
        // => 13:30:22
    }
}