Core/Calendar: Implement different timezone support for ingame calendar

Port From (https://github.com/TrinityCore/TrinityCore/commit/b888b1b09f71a8b8b4a9d45c804a1f164fb65ac3)

Source/Game/Handlers/AuthenticationHandler.cs

@@ -1,8 +1,10 @@
 // Copyright (c) CypherCore <http://github.com/CypherCore> All rights reserved.
 // Licensed under the GNU GENERAL PUBLIC LICENSE. See LICENSE file in the project root for full license information.
 
+using Framework;
 using Framework.Constants;
 using Game.Networking.Packets;
+using System;
 
 namespace Game
 {
@@ -68,11 +70,14 @@ namespace Game
 
         public void SendSetTimeZoneInformation()
         {
-            // @todo: replace dummy values
+            TimeSpan timezoneOffset = Timezone.GetSystemZoneOffset();
+            string realTimezone = Timezone.GetSystemZoneName();
+            string clientSupportedTZ = Timezone.FindClosestClientSupportedTimezone(realTimezone, timezoneOffset);
+
             SetTimeZoneInformation packet = new();
-            packet.ServerTimeTZ = "Europe/Paris";
-            packet.GameTimeTZ = "Europe/Paris";
-            packet.ServerRegionalTZ = "Europe/Paris";
+            packet.ServerTimeTZ = clientSupportedTZ;
+            packet.GameTimeTZ = clientSupportedTZ;
+            packet.ServerRegionalTZ = clientSupportedTZ;
 
             SendPacket(packet);//enabled it
         }