Understanding C# threading interview question

Question

I was recently asked a thread safety question in a c# interview. I didn't get it quite right. I'm am trying to understand it now. Here's the question...

Given a basic Order class...

using System;

namespace OrderThreadTest
{
    public class Order
    {
        public TimeSpan CancelTime;
        public int Id;
    }
}

Given a simple schedule class stub however I have implemented it to test at home...

using System;
using System.Threading;

namespace OrderThreadTest
{
    public class Scheduler : IDisposable
    {
        private Timer _timer;

        public IDisposable Schedule(Order order, Action action)
        {
            var current = DateTime.Now;
            var timeToGo = order.CancelTime - current.TimeOfDay;

            _timer = new Timer(x => { action(); }, null, timeToGo, Timeout.InfiniteTimeSpan);

            return this;
        }

        public void Dispose()
        {
        }
    }
}

How do you ensure that the Cancel method in the worker class is thread safe? My answer is the commented out pieces

using System;

namespace OrderThreadTest
{
    public class Answer
    {
        private readonly Scheduler _scheduler;
        private object _myLock = new object();
        private Order _order;

        public Answer(Scheduler scheduler)
        {
            _scheduler = scheduler;
        }

        public void Cancel(Order order)
        {
            //  lock (_myLock)
            //  {
            // _order = order;
            //  var result =
            _scheduler.Schedule(order, () =>
            {
                //if (order.Equals(_order))
                //{
                Console.WriteLine("Canceled: " + order.Id);
                order = null;
                //}
            });
            //  }
        }
    }
}

My first question in understanding this is how can I cause an example of a second thread setting the passed in Order and changing a earlier timer? For example cause a thread data clash.

I have tried like this but it always seems to run as expected...

using System;

namespace OrderThreadTest
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            var a1 = new Answer(new Scheduler());

            var o = new Order
                {Id = 1, CancelTime = new TimeSpan(DateTime.Now.Hour, DateTime.Now.Minute, DateTime.Now.Second + 5)};

            a1.Cancel(o);

            a1.Cancel(o);

            Console.ReadLine();
        }
    }
}

How do I repro the problem that I am meant to solve here?