Core/Calendar: Implement different timezone support for ingame calendar

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

Source/Framework/Time/Time.cs

@@ -93,7 +93,7 @@ public static class Time
 
     public static DateTime UnixTimeToDateTime(long unixTime)
     {
-        return DateTimeOffset.FromUnixTimeSeconds(unixTime).LocalDateTime;
+        return DateTimeOffset.FromUnixTimeSeconds(unixTime).DateTime;
     }
 
     public static long DateTimeToUnixTime(DateTime dateTime)

Source/Framework/Time/Timezone.cs

@@ -0,0 +1,78 @@
+// 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 System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace Framework
+{
+    public class Timezone
+    {
+        static Dictionary<uint, TimeSpan> _timezoneOffsetsByHash = InitTimezoneHashDb();
+
+        static Dictionary<uint, TimeSpan> InitTimezoneHashDb()
+        {
+            // Generate our hash db to match values sent in client authentication packets
+            Dictionary<uint, TimeSpan> hashToOffset = new();
+            foreach (var sysInfo in TimeZoneInfo.GetSystemTimeZones())
+            {
+                TimeSpan offsetMinutes = sysInfo.BaseUtcOffset;
+                hashToOffset.TryAdd(offsetMinutes.TotalMinutes.ToString().HashFnv1a(), offsetMinutes);
+            }
+
+            return hashToOffset;
+        }
+
+        static (TimeSpan, string)[] _clientSupportedTimezones =
+        {
+            (TimeSpan.FromMinutes(-480), "America/Los_Angeles"),
+            ( TimeSpan.FromMinutes(-420), "America/Denver" ),
+            ( TimeSpan.FromMinutes(-360), "America/Chicago" ),
+            ( TimeSpan.FromMinutes(-300), "America/New_York" ),
+            ( TimeSpan.FromMinutes(-180), "America/Sao_Paulo" ),
+            ( TimeSpan.FromMinutes(0), "Etc/UTC" ),
+            ( TimeSpan.FromMinutes(60), "Europe/Paris" ),
+            ( TimeSpan.FromMinutes(480), "Asia/Shanghai" ),
+            ( TimeSpan.FromMinutes(480), "Asia/Taipei" ),
+            ( TimeSpan.FromMinutes(540), "Asia/Seoul" ),
+            ( TimeSpan.FromMinutes(600), "Australia/Melbourne" ),
+        };
+
+        public static TimeSpan GetOffsetByHash(uint hash)
+        {
+            if (_timezoneOffsetsByHash.TryGetValue(hash, out var offset))
+                return offset;
+
+            return TimeSpan.Zero;
+        }
+
+        public static TimeSpan GetSystemZoneOffsetAt(DateTime date)
+        {
+            return TimeZoneInfo.Local.GetUtcOffset(date);
+        }
+
+        public static TimeSpan GetSystemZoneOffset()
+        {
+            return DateTimeOffset.Now.Offset;
+        }
+
+        public static string GetSystemZoneName()
+        {
+            return TimeZoneInfo.Local.StandardName;
+        }
+
+        public static string FindClosestClientSupportedTimezone(string currentTimezone, TimeSpan currentTimezoneOffset)
+        {
+            // try exact match
+            var pair = _clientSupportedTimezones.FirstOrDefault(tz => tz.Item2 == currentTimezone);
+            if (!pair.Item2.IsEmpty())
+                return pair.Item2;
+
+            // try closest offset
+            pair = _clientSupportedTimezones.MinBy(left => left.Item1 - currentTimezoneOffset);
+
+            return pair.Item2;
+        }
+    }
+}

Source/Framework/Util/Extensions.cs

@@ -422,6 +422,17 @@ namespace System
 
             return -1;
         }
+
+        public static uint HashFnv1a(this string data)
+        {
+            uint hash = 0x811C9DC5u;
+            foreach (char c in data)
+            {
+                hash ^= c;
+                hash *= 0x1000193u;
+            }
+            return hash;
+        }
         #endregion
 
         #region BinaryReader