Not able to compare Json subitem datetime with current datetime in c#view

Question

I am using azure cosmosdb database. I am displaying webpage using Cosmosdb data and c# MVC web application. This time database json filr have DateTime as subitem in Json. I need to compare it with current datetime to flag signal on view page. Please find attached source code.

if I use  "DateTime myday =@item.LastContact.Ping;"

I am getting error:
Object reference not set to an instance of an object.

---

if I use " DateTime myday = Html.DisplayFor(modelItem => item.LastContact.Ping);" I get error:
Cannot implicitly convert type 'System.Web.Mvc.MvcHtmlString' to 'System.DateTime'

Can anybody help please.

My code is as follows:

JSON:

 "DataTypeId": 1,
"IPaddress": "192.168.2.177",
"id": "d1b81653-b7a5-4d79-85ea-79c01f43f747",
"PhoneNo": "0417518324",
"LastContact": {
    "EnvLog": "2019-09-07T19:41:08",
    "Ping": "2020-01-09T12:10:09",
    "SpdLog": "2019-09-07T19:41:08"
},
"SpeedLog": true,
"DeviceModelId": 2,
"DisconnectEvents": 9,

Model:

    public class DeviceMap
{

    [JsonProperty(PropertyName = "id")]
    public string Id { get; set; }

    [JsonProperty(PropertyName = "IPaddress")]
    public string IPaddress { get; set; }

    [JsonProperty(PropertyName = "SpeedLog")]
    public bool SpeedLog { get; set; }

    [JsonProperty(PropertyName = "LastContact")]
    public Entry LastContact { get; set; }
}

 public class Entry

{

    [JsonProperty(PropertyName = "EnvLog")]
    public DateTime EnvLogtime { get; set; }

    [JsonProperty(PropertyName = "Ping")]
    public DateTime Ping { get; set; }

    [JsonProperty(PropertyName = "SpdLog")]
    public DateTime SpdLog { get; set; }
}

}

Controller:

public class DeviceMapController : Controller
{
    [ActionName("Index")]
    public async Task<ActionResult> Map1Async()
    {
        string empty = "Unknown";

        var items = await DocumentDBRepository<DeviceMap>.GetMap1Async(d => d.Id != null, d => d.Lat != empty);
        if (Convert.ToInt32(User.GroupId()) != 200) 
            {
                items = items.Where(d => d.GroupId == Convert.ToInt32(User.GroupId()));

        }

        return View(items);

    }

}

View:

         @foreach (var item in Model)
                    {
 if ((@item.IPaddress == "192.168.2.14") || (@item.IPaddress == "192.168.2.13") || (@item.IPaddress == "192.168.2.15") || (@item.IPaddress == "192.168.2.16")) { }
                        else
                        {

                            //SideId string
                            string siteId = null;
                            string newsiteId = null;
                            if (item.IPaddress != "0")
                            {

                                int index = item.IPaddress.IndexOf('.') + 1;
                                int space = item.IPaddress.IndexOf('.', index + 1);
                                int space2 = item.IPaddress.IndexOf('.', space + 1);
                                int end = item.IPaddress.Length;
                                if (index > 0) { siteId = item.IPaddress.Substring(space + 1, end - space - 1); newsiteId = siteId.Replace('.', '-'); }
                            }
                           //  string Cutoff =Html.DisplayFor(modelItem => item.LastContact.Ping).ToString();
                         // DateTime day = DateTime.Parse(Cutoff.Value,"dd/MM/yyyy HH:mm:ss", CultureInfo.CurrentCulture);

                          DateTime myday =Html.DisplayFor(modelItem => item.LastContact.Ping);
                            DateTime now = DateTime.Now;
                            DateTime yesterday = now.AddDays(-7);
                            DateTime d;
                       //  DateTime.TryParseExact(myday,@"dd/MM/yyyy HH:mm:ss", CultureInfo.InvariantCulture, DateTimeStyles.AssumeUniversal,out d);


                          // if (Html.DisplayFor(modelItem => item.LastContact.Ping) != null) { d = DateTime.Parse(myday, "dd/MM/yyyy HH:mm:ss", CultureInfo.CurrentCulture); } else { myday = DateTime.ParseExact("08/04/2000 14:00:02", "dd/MM/yyyy HH: mm:ss", CultureInfo.CurrentCulture); }

                            string a = @item.Name;



                            <tr>

                                <td>

                                    @if (a.Length > 50) { a = a.Substring(0, 50); }



                                        @if (item.GroupId != 82)
                                        {

                                            if (myday <= DateTime.Now.AddDays(-7))
                                            {<h4>@myday</h4>
                                                    <img src="/Content/Images/trafficlight-red2.png" width="20" height="20"><a href="@Url.Action("DevicePage", "Device", new { item.DeviceId })"><h6>@a..</h6></a>
                                            }
                                            else
                                            {<img src="/Content/Images/trafficlight-green1.png" width="20" height="20"><a href="@Url.Action("DevicePage", "Device", new { item.DeviceId })"><h6>@a..</h6></a>
                                            }
                                        }


                                        else
                                        {


                                            if (myday <= DateTime.Now.AddDays(-7))
                                            {
                                                <img src="/Content/Images/trafficlight-red2.png" width="20" height="20"><a href="@Url.Action("Form", "TARP", new { item.DeviceId })"><h6>@a..</h6></a>
                                            }
                                            else
                                            {<img src="/Content/Images/trafficlight-green1.png" width="20" height="20"><a href="@Url.Action("Form", "TARP", new { item.DeviceId })"><h6>@a..</h6></a>

                                            }

                                        }

                                        @if (item.IPaddress != "0")
                                        {<p>Site Id: @newsiteId;</p>}
                                    @if (item.LastPing != null)
                                    {
                                        <p>
                                            Last Contact: @Html.DisplayFor(modelItem => item.LastContact.Ping), @Html.DisplayFor(modelItem => item.BatVoltage)V

                                        </p>}