Core/Calendar: Improve calendar timezone handling

Port From (https://github.com/TrinityCore/TrinityCore/commit/5600316c9f69d2afda82ae0e632715b420dfd402)

Source/Framework/Util/Time.cs

@@ -141,6 +141,11 @@ public static class Time
         return hourLocal;
     }
 
+    public static long LocalTimeToUTCTime(long time)
+    {
+        return DateTimeToUnixTime(UnixTimeToDateTime(time).ToUniversalTime());
+    }
+
     public static string secsToTimeString(ulong timeInSecs, TimeFormat timeFormat = TimeFormat.FullText, bool hoursOnly = false)
     {
         ulong secs = timeInSecs % Minute;

Source/Game/Handlers/CalendarHandler.cs

@@ -121,6 +121,8 @@ namespace Game
         {
             ObjectGuid guid = GetPlayer().GetGUID();
 
+            calendarAddEvent.EventInfo.Time = Time.LocalTimeToUTCTime(calendarAddEvent.EventInfo.Time);
+
             // prevent events in the past
             // To Do: properly handle timezones and remove the "- time_t(86400L)" hack
             if (calendarAddEvent.EventInfo.Time < (GameTime.GetGameTime() - 86400L))
@@ -204,6 +206,8 @@ namespace Game
             ObjectGuid guid = GetPlayer().GetGUID();
             long oldEventTime;
 
+            calendarUpdateEvent.EventInfo.Time = Time.LocalTimeToUTCTime(calendarUpdateEvent.EventInfo.Time);
+
             // prevent events in the past
             // To Do: properly handle timezones and remove the "- time_t(86400L)" hack
             if (calendarUpdateEvent.EventInfo.Time < (GameTime.GetGameTime() - 86400L))
@@ -240,6 +244,8 @@ namespace Game
         {
             ObjectGuid guid = GetPlayer().GetGUID();
 
+            calendarCopyEvent.Date = Time.LocalTimeToUTCTime(calendarCopyEvent.Date);
+
             // prevent events in the past
             // To Do: properly handle timezones and remove the "- time_t(86400L)" hack
             if (calendarCopyEvent.Date < (GameTime.GetGameTime() - 86400L))