Problem with Session when getting string data to the action of controller

Hi guys,

As some of you knows, I'm developing a quiz application. I have timer in question view and this time I need to get the time between opening the question view and submitting the answer. Then I substruct that interval from 60 in order to get remained seconds which will be the point of the question if the answer is right.  Here is the details:

View

@model Intellect.Models.ViewModels.AdminViewModel<div class="questioncontainer">        <form asp-action="Question" asp-controller="Home" asp-route-id="@Model.NextQuestion.Id" asp-route-count="@ViewBag.Equestions"><div class="row"><div class="col-lg-3"></div><div class="col-lg-6 col-sm-12"><table><tr><th>Qaliq vaxt</th></tr><tr><td><div id="time"></div> </td></tr></table><div class="question">@Model.CurrentQuestion.Description </div></div><div class="col-lg-3"></div></div><div class="row"><div class="col-lg-3 col-sm-0"></div>
                @{
                    int n = 0;
                }
                @foreach (Answer item in Model.Answers)
                {
                    if (n == 0 || n == 2)
                    {
                        @: <div class="col-lg-3 col-sm-12">
                            @: <div class="firstpart">
                            }<input asp-for="@item.Id" name="@item.Id" hidden /><input type="radio" asp-for="@item.Id" name="myanswer" value="@item.Id" />@item.Description<br>
                            if (n == 1 || n == 3)
                            {
                            @:</div>
                        @:</div>
                    }

                    n++;
                }<div class="col-lg-3"></div></div><div class="row"><div class="col-lg-6 col-sm-4"></div><div class="col-lg-3 col-sm-4"></div><div class="col-lg-3 col-sm-4"><div class="nextbtn">
                        @if (ViewBag.Equestions == 0)
                        {<input type="submit" value="Finish" />
                        }
                        else
                        {<input type="submit" value="Next" />
                        }</div></div></div></form></div>

@section Script{ 
    <script>
        function StartTimer(seconds) {
            var intSeconds = seconds;
            var timer = setInterval(myTimer, 1000);
            function myTimer() {
                if (intSeconds < 0) {
                    Alert("bitdi")
                    clearInterval(timer)
                    return;
                }
                document.getElementById("time").innerText = intSeconds;
                intSeconds--;
            }
        }
        StartTimer(60);</script>
}

Controller

 public class HomeController : Controller
    {
        private readonly IntellectDbContext _intellectDbContext;
        private readonly UserManager<ExamUser> _userManager;
        private readonly SignInManager<ExamUser> _signInManager;
        static int exam_id = 0;
        static int? PreviousId = 0;
        static int result = 0;
        static int correctAnswer = 0;
        static List<Question> RemainedQuestions = new List<Question>();
        static List<int> trueAnswers = new List<int>();

        public HomeController(IntellectDbContext intellectDbContext, UserManager<ExamUser> userManager, SignInManager<ExamUser> signInManager)
        {
            _intellectDbContext = intellectDbContext;
            _userManager = userManager;
            _signInManager = signInManager;
        }
        [HttpGet]
        public async Task<IActionResult> Test(int Id)
        {
            exam_id = Id;
            AdminViewModel admodel = new AdminViewModel();
            admodel.Equestions = await _intellectDbContext.Questions.Include(q => q.Answers).Where(q => q.ExamId == Id).ToListAsync();
            admodel.CurrentQuestion = await _intellectDbContext.Questions.Where(q => q.ExamId == Id).FirstOrDefaultAsync();
            RemainedQuestions = admodel.Equestions;
            PreviousId = admodel.CurrentQuestion.Id;
            return View(admodel);
        }


        [HttpGet]
        public async Task<IActionResult> Question(int Id, int count)
        {
            AdminViewModel admodel = new AdminViewModel();
            admodel.CurrentQuestion = await _intellectDbContext.Questions.Where(x => x.Id == Id).SingleOrDefaultAsync();
            admodel.Answers = await _intellectDbContext.Answers.Where(y => y.QuestionId == Id).ToListAsync();
            admodel.Equestions = await _intellectDbContext.Questions.Where(q => q.ExamId == exam_id).ToListAsync();

            // admodel.CurrentQuestion.Remainedtime = new TimeSpan(0, 0, 60);
            HttpContext.Session.SetString("currentQuestion_" + admodel.CurrentQuestion.Id, DateTime.Now.ToString());


            if (count > 1)
            {
                var question = RemainedQuestions.Single(r => r.Id == admodel.CurrentQuestion.Id);
                PreviousId = question.Id;
                RemainedQuestions.Remove(question);
                admodel.NextQuestion = RemainedQuestions[0];
                count -= 1;
            }
            else
            {
                admodel.NextQuestion = RemainedQuestions[0];
                count -= 1;
            }

            if (count == -1)
            {
                return RedirectToAction(nameof(Finish));
            }

            ViewBag.Equestions = count;

            return View(admodel);
        }

        [HttpPost]
        public async Task<IActionResult> Question(int Id, int count, int myanswer)
        {
            AdminViewModel admodel = new AdminViewModel();
            admodel.CurrentQuestion = await _intellectDbContext.Questions.Where(x => x.Id == Id).SingleOrDefaultAsync();
            admodel.Answers = await _intellectDbContext.Answers.Where(y => y.QuestionId == Id).ToListAsync();
            admodel.Equestions = await _intellectDbContext.Questions.Where(q => q.ExamId == exam_id).ToListAsync();

            //  admodel.CurrentQuestion.Remainedtime = new TimeSpan(0, 0, 60);
           // HttpContext.Session.SetString("currentQuestion_" + admodel.CurrentQuestion.Id, DateTime.Now.ToString());
            var interval = DateTime.Now - DateTime.Parse(HttpContext.Session.GetString("currentQuestion_" + Id));

            correctAnswer = _intellectDbContext.Answers.Where(a => a.QuestionId == PreviousId && a.Correct == true).SingleOrDefault().Id;

            if (_signInManager.IsSignedIn(User))
            {
                ExamUser examTaker = await _userManager.GetUserAsync(HttpContext.User);

                examTaker.TestTaker = await _intellectDbContext.TestTakers
                                               .Include(tt => tt.UserQuestions) 
                                               .Where(t => t.Id == examTaker.TestTakerId) 
                                               .FirstOrDefaultAsync();
                admodel.CurrentQuestion = examTaker.TestTaker.UserQuestions.Select(u => u.Question).Where(x => x.Id == Id).SingleOrDefault();
                if (myanswer == correctAnswer)
                {
                    admodel.CurrentQuestion.Score = 60 - (int)interval.TotalMilliseconds / 1000;
                    await _intellectDbContext.SaveChangesAsync();
                    // admodel.CurrentQuestion.Score = result;
                }
            }
                if (count > 1)
            {
                var question = RemainedQuestions.Single(r => r.Id == admodel.CurrentQuestion.Id);
                PreviousId = question.Id;
                RemainedQuestions.Remove(question);
                admodel.NextQuestion = RemainedQuestions[0];
                count -= 1;
            }
            else
            {
                admodel.NextQuestion = RemainedQuestions[0];
                count -= 1;
            }

            if(count == -1)
            {
                return RedirectToAction(nameof(Finish));
            }

            ViewBag.Equestions = count;

            return RedirectToAction(nameof(Question));
        }
     }

When I run the application, the following errors comes (refer to link) when clicking on Next to get to the next question view:  https://prnt.sc/s6y9vy

As you see, action (Question) in controller cannot get string data from session to parse it. 

The problem lies here:        var interval = DateTime.Now - DateTime.Parse(HttpContext.Session.GetString("currentQuestion_" + Id));

Please, assist in solving this issue.