using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var offsets = GetOffsets(simplified: true); // toggle parameter false to see all offsets

        DisplayOffsets(offsets);

    }

​

    public static IEnumerable<TimeSpan> GetOffsets(bool simplified)

    {

        return simplified ? GetSimplifiedOffsets() : GetAllOffsets();

    }

​

    private static IEnumerable<TimeSpan> GetSimplifiedOffsets()

    {

        var currentYear = DateTime.UtcNow.Year;

        var offsets = new HashSet<TimeSpan>();

        foreach (var tz in TimeZoneInfo.GetSystemTimeZones())

        {

            var yearsToTest = new HashSet<int>{currentYear};

            foreach (var rule in tz.GetAdjustmentRules())

            {

                yearsToTest.Add(rule.DateStart.Year);

            }

​

            foreach (var year in yearsToTest)

            {

                var o1 = tz.GetUtcOffset(new DateTime(year, 1, 1));

                var o2 = tz.GetUtcOffset(new DateTime(year, 7, 1));

                if (o1.Ticks % TimeSpan.TicksPerMinute == 0)

                    offsets.Add(o1);

                if (o2.Ticks % TimeSpan.TicksPerMinute == 0)

                    offsets.Add(o2);

            }

        }

​

        return offsets.OrderBy(x => x);

    }

​

    private static IEnumerable<TimeSpan> GetAllOffsets()

    {

        var min = TimeSpan.FromHours(-14);

        var max = TimeSpan.FromHours(14);

        var interval = TimeSpan.FromMinutes(1);

        for (var o = min; o <= max; o = o.Add(interval))

        {

            yield return o;

        }

    }

​

    private static void DisplayOffsets(IEnumerable<TimeSpan> offsets)

    {

        foreach (var offset in offsets)

        {

            var sign = offset < TimeSpan.Zero ? "-" : "+";

            Console.WriteLine("{0}{1:hh\\:mm}", sign, offset);

        }

    }

}