Use IGameTiming for lobby timing instead of DateTime.

Fixes #1681

Content.Client/Interfaces/IClientGameTicker.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using Robust.Shared.Network;
+using Robust.Shared.Timing;
 using static Content.Shared.GameTicking.SharedGameTicker;
 
 namespace Content.Client.Interfaces
@@ -11,7 +12,7 @@ namespace Content.Client.Interfaces
         string ServerInfoBlob { get; }
         bool AreWeReady { get; }
         bool DisallowedLateJoin { get; }
-        DateTime StartTime { get; }
+        TimeSpan StartTime { get; }
         bool Paused { get; }
         Dictionary<NetUserId, PlayerStatus> Status { get; }
         IReadOnlyList<string> JobsAvailable { get; }