How to fill the dropdown with all timezone

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C#

I have question about timezone, i want to fill the dropdown with all timezone in world and when i select one of them i want to save the selected value of that dropdown, for that i use the code

ReadOnlyCollection<TimeZoneInfo> timezoneInfo;
timezoneInfo = TimeZoneInfo.GetSystemTimeZones();
strTimezone = string.Join(";", from Time in timezoneInfo
select string.Format("{0}:{1}",
Time.Id, Time.DisplayName));

but with the help of that my dropdown is fill with only one timezone which is chennai kolkatta because my system has only one timezone which is chennai kolkatta that dropdown contain only one value, but i want all timezone for that what i do please help me as early as possible.