I found that other module (like repository) has same issue. And It seems that each account's preferred timezone setting does not works for module.

You will likely want to recompile the forum source code with the ConvertTimeZone method in the utility class to be changed to 

Dim postTime As DateTime = displayCreatedDate
                Dim GMTTime As DateTime = System.TimeZoneInfo.ConvertTimeToUtc(displayCreatedDate, GetPortalSettings().TimeZone)
If HttpContext.Current.Request.IsAuthenticated Then
Dim objUser As Users.UserInfo = Users.UserController.GetCurrentUserInfo
                    'Dim UserTimeZone As System.TimeZoneInfo = objUser.Profile.PreferredTimeZone
                    'Dim UserTime As DateTime = System.TimeZoneInfo.ConvertTime(displayCreatedDate, UserTimeZone)
                    'displayCreatedDate = UserTime

                    displayCreatedDate = objUser.LocalTime(GMTTime)
End If

that is what I had to do to get it working with DNN6 time zone info for user profiles.