How to set timer of 5 min to do certain operation in app and if timeouts, raise another event to do other task

The timer is fine, I believe.
To cancel the reservation, make sure a single method is in charge to completely commit it. This way, until such method has not been called your code may cancel the reservation without having to perform backward steps.

Timer is actually a C# object (or type) that you can use in your C# programs and applications, such as WPF, WinForms, etc. But since you have tagged MVC 5 with this question and most probably you want to implement this feature in a web application, the best approach would be to control this from client-side.

In JavaScript, you can do this,

/* 
 * bookingMade to be set to true if user has made a booking so timer 
 * can be ignored and user can continue, in case timer hits. 
 */
let bookingMade = false;
let minutes = 5 * 60 * 1000; // 5 minutes

setTimeout(function() {
    if(!bookingMade) {
        // Refresh the page and capture a new session
    }
    // else is not needed, as per your case.
}, minutes); // Attach the timeout

This way, you can control the requests from the client side and reject any request that comes after 5 minutes. Of course, this is just off the top of my head right now and you can do a better job than me. But in the nutshell this is the way that you need to take, in order to control this reservation system.

See here for more information on how to control this timer, WindowOrWorkerGlobalScope.setTimeout() - Web APIs | MDN[^]

A server-side implementation of session is also a good approach, since your server will itself reject any request that comes once the session has timed out, but I have omitted that part out since sessions typically do not follow timers—each time a request is made, a new instance is created and thus will force you to create a static instance of the timer and maintain it along with the request or session information, which is quite ugly as I imagine it—and you might have to write session timeouts yourself, see here Session timeout in ASP.NET - Stack Overflow[^]. This server-side implementation can control most of the stuff that you would have to implement yourself.

I found this code and it worked exactly the way I wanted.

private static void OnTimedEvent(object source, ElapsedEventArgs e)
    { 
        //do something
    }

    protected void Button1_Click(object sender, EventArgs e)
    {

        if (Session["Timer"] == null)
        {
            Session["Timer"] = new System.Timers.Timer();
        }
        
        ((System.Timers.Timer)Session["Timer"]).Interval = 5000;
        ((System.Timers.Timer)Session["Timer"]).Elapsed += OnTimedEvent;
        ((System.Timers.Timer)Session["Timer"]).AutoReset = true;
        ((System.Timers.Timer)Session["Timer"]).Enabled = true; 
    }

    protected void Button3_Click(object sender, EventArgs e)
    {
        ((System.Timers.Timer)Session["Timer"]).Enabled = false;
    }

This is the code I found in msdn blog. This is the best thing I found for me. I will be using this::

using System;
using System.Timers;
namespace Timer
{
    public partial class Index : System.Web.UI.Page
    {
        private static System.Timers.Timer aTimer;

        protected void Page_Load(object sender, EventArgs e)
        {          

        }

        // Specify what you want to happen when the Elapsed event is raised.
        private static void OnTimedEvent(object source, ElapsedEventArgs e)
        { 

        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            // Create a timer and set a two second interval.
            aTimer = new System.Timers.Timer();
            aTimer.Interval = 5000; //10 sec

            // Hook up the Elapsed event for the timer. 
            aTimer.Elapsed += OnTimedEvent;

            // Have the timer fire repeated events (true is the default)
            aTimer.AutoReset = true; //AutoReset = false raises ElapsedEventHandler only once

            // Start the timer
            aTimer.Enabled = true; 
        }

        protected void Button2_Click(object sender, EventArgs e)
        {

            for (int i=0; i<100000000; i++)
            {
                System.Threading.Thread.Sleep(1000);
            }
            //aTimer.Enabled = true;
        }

        protected void Button3_Click(object sender, EventArgs e)
        {
            aTimer.Enabled = false;
        }
    }
}