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DateTime dt = DateTime.Parse(data_shanghai);  
bool dtttt = dt.Equals(TimeZone.CurrentTimeZone.GetDaylightChanges(dt.Year).Start);
            if (dtttt == true)
            {
                MessageBox.Show(dt + " ist Sommerzeit");
                return;
            }
            else
            {
                MessageBox.Show(dt + " ist keine Sommerzeit");
                return;
            }                    

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private void Worldtimer(object sender, EventArgs e)
        {
            string displayShanghai = "(GMT+07:00) China/Shanghai Time";
            string displayBerlin = "(GMT+01:00) Deutschland/Berlin Time";
            string displayLondon = "(GMT+00:00) England/London Time";
            string displaySeoul = "(GMT+06:00) Korea/Seoul Time";
            string displaySydney = "(GMT+11:00) Australien/Sydney Time";
            string displayNewYork = "(GMT-05:00) America/New York Time";

            string standard_shanghai = "Shanghai Time";
            string standard_berlin = "Berlin Time";
            string standard_london = "London Time";
            string standard_seoul = "Seoul Time";
            string standard_sydney = "Sydney Time";
            string standard_newyork = "New York Time";

            TimeSpan shanghai_offset = new TimeSpan(07, 00, 00);
            TimeSpan berlin_offset = new TimeSpan(01, 00, 00);
            TimeSpan london_offset = new TimeSpan(00, 00, 00);
            TimeSpan seoul_offset = new TimeSpan(06, 00, 00);
            TimeSpan sydney_offset = new TimeSpan(11, 00, 00);
            TimeSpan newyork_offset = new TimeSpan(-05, 00, 00);

            TimeZoneInfo str_Shanghai = TimeZoneInfo.CreateCustomTimeZone(standard_shanghai, shanghai_offset, displayShanghai, standard_shanghai);
            TimeZoneInfo str_Berlin = TimeZoneInfo.CreateCustomTimeZone(standard_berlin, berlin_offset, displayBerlin, standard_berlin);
            TimeZoneInfo str_London = TimeZoneInfo.CreateCustomTimeZone(standard_london, london_offset, displayLondon, standard_london);
            TimeZoneInfo str_Seoul = TimeZoneInfo.CreateCustomTimeZone(standard_seoul, seoul_offset, displaySeoul, standard_seoul);
            TimeZoneInfo str_Sydney = TimeZoneInfo.CreateCustomTimeZone(standard_sydney, sydney_offset, displaySydney, standard_sydney);
            TimeZoneInfo str_Newyork = TimeZoneInfo.CreateCustomTimeZone(standard_newyork, newyork_offset, displayNewYork, standard_newyork);

            string data_shanghai = TimeZoneInfo.ConvertTime(DateTime.Now, TimeZoneInfo.Local, str_Shanghai).ToString();
            string data_berlin = TimeZoneInfo.ConvertTime(DateTime.Now, TimeZoneInfo.Local, str_Berlin).ToString();
            string data_london = TimeZoneInfo.ConvertTime(DateTime.Now, TimeZoneInfo.Local, str_London).ToString();
            string data_seoul = TimeZoneInfo.ConvertTime(DateTime.Now, TimeZoneInfo.Local, str_Seoul).ToString();
            string data_sydney = TimeZoneInfo.ConvertTime(DateTime.Now, TimeZoneInfo.Local, str_Sydney).ToString();
            string data_newyork = TimeZoneInfo.ConvertTime(DateTime.Now, TimeZoneInfo.Local, str_Newyork).ToString();

            string[] substr1 = Regex.Split(data_shanghai, " ");
            string[] substr2 = Regex.Split(data_berlin, " ");
            string[] substr3 = Regex.Split(data_london, " ");
            string[] substr4 = Regex.Split(data_seoul, " ");
            string[] substr5 = Regex.Split(data_sydney, " ");
            string[] substr6 = Regex.Split(data_newyork, " ");

            switch (toolStripComboBox_world.SelectedIndex)
            {
                case 0: // Berlin
                    Label_time.Text = substr2.GetValue(1).ToString();
                    Label_date.Text = substr2.GetValue(0).ToString();
                    break;
                case 1: // London
                    Label_time.Text = substr3.GetValue(1).ToString();
                    Label_date.Text = substr3.GetValue(0).ToString();
                    break;
                case 2: // Shanghai
                    Label_time.Text = substr1.GetValue(1).ToString();
                    Label_date.Text = substr1.GetValue(0).ToString();
                    break;
                case 3: // Seaoul
                    Label_time.Text = substr4.GetValue(1).ToString();
                    Label_date.Text = substr4.GetValue(0).ToString();
                    break;
                case 4: // Sydney
                    Label_time.Text = substr5.GetValue(1).ToString();
                    Label_date.Text = substr5.GetValue(0).ToString();
                    break;
                case 5: // New York
                    Label_time.Text = substr6.GetValue(1).ToString();
                    Label_date.Text = substr6.GetValue(0).ToString();
                    break;
                default: // Standarttime
                    Label_time.Text = DateTime.Now.ToShortTimeString();
                    Label_date.Text = DateTime.Now.ToShortDateString();
                    break;
            }
            
        }                    

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var tzi = TimeZoneInfo.FindSystemTimeZoneById("China Standard Time");
Console.WriteLine("UTC: {0} Lokal: {1} {2}: {3}", DateTime.UtcNow, DateTime.Now, tzi.StandardName, TimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, tzi));                    

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            TimeZoneInfo timeZoneInfo;
            DateTime dateTime;

            timeZoneInfo = TimeZoneInfo.FindSystemTimeZoneById("China Standard Time");

            dateTime = TimeZoneInfo.ConvertTime(DateTime.Now, timeZoneInfo);
            listBox1.Items.Add(dateTime);                    

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using System.Collections.ObjectModel; 

ReadOnlyCollection<TimeZoneInfo> zones = TimeZoneInfo.GetSystemTimeZones();
            foreach (TimeZoneInfo zone in zones)
               listBox1.Items.Add(zone.Id);                    

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label1.Text = dateTime.ToString("t");