This is a quick console application which you can adapt to your ASP.NET page.
using System;
namespace TimeDemo
{
class Program
{
static void Main(string[] args)
{
DateTime now = DateTime.Now;
DateTime earlier = now.AddHours(-1);
Console.WriteLine("Now: {0}", now.ToLongTimeString());
Console.WriteLine("Earlier: {0}", earlier.ToLongTimeString());
TimeSpan duration = now - earlier;
Console.WriteLine("Duration {0} minutes", duration.TotalMinutes);
Console.ReadKey();
}
}
}