Fix crash when getting server status

- Adds _roundStartDateTime field to GameTicker, the UTC time when the round started.

Content.Server/GameTicking/GameTicker.StatusShell.cs

@@ -1,6 +1,8 @@
+using System;
 using Newtonsoft.Json.Linq;
 using Robust.Server.ServerStatus;
 using Robust.Shared.IoC;
+using Robust.Shared.ViewVariables;
 
 namespace Content.Server.GameTicking
 {
@@ -11,6 +13,12 @@ namespace Content.Server.GameTicking
         /// </summary>
         private readonly object _statusShellLock = new();
 
+        /// <summary>
+        ///     Round start time in UTC, for status shell purposes.
+        /// </summary>
+        [ViewVariables]
+        private DateTime _roundStartDateTime;
+
         private void InitializeStatusShell()
         {
             IoCManager.Resolve<IStatusHost>().OnStatusRequest += GetStatusResponse;
@@ -26,7 +34,7 @@ namespace Content.Server.GameTicking
                 jObject["run_level"] = (int) _runLevel;
                 if (_runLevel >= GameRunLevel.InRound)
                 {
-                    jObject["round_start_time"] = _roundStartTime.ToString("o");
+                    jObject["round_start_time"] = _roundStartDateTime.ToString("o");
                 }
             }
         }