ASP.NET

using System.Web;
using System.Text;
using System.Web.UI;

/// <summary>
/// A JavaScript alert
/// </summary>
public static class Alert
{

/// <summary>
/// Shows a client-side JavaScript alert in the browser.
/// </summary>
/// <param name="message">The message to appear in the alert.</param>
public static void Show(string message)
{
   // Cleans the message to allow single quotation marks
   string cleanMessage = message.Replace("'", "\'");
   string script = "<script type="text/javascript">alert('" + cleanMessage + "');</script>";

   // Gets the executing web page
   Page page = HttpContext.Current.CurrentHandler as Page;

   // Checks if the handler is a Page and that the script isn't allready on the Page
   if (page != null && !page.ClientScript.IsClientScriptBlockRegistered("alert"))
   {
     page.ClientScript.RegisterClientScriptBlock(typeof(Alert), "alert", script);
   }
}
} 

void btnSave_Click(object sender, EventArgs e)
{
   try
   {
     SaveSomething();
     Alert.Show("You document has been saved");
   }
   catch (ReadOnlyException)
   {
     Alert.Show("You do not have write permission to this file");
   }
}

TimeSpan difference = myFirstDate - mySecondDate;