using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        // For the UK, use "Europe/London" (or "GMT Standard Time" on Windows before .NET 6)

        TimeZoneInfo tz = TimeZoneInfo.FindSystemTimeZoneById("Europe/London");

        DateTime london = new DateTime(2023, 10, 29, 1, 30, 0);

        DateTime utcFromDst = london.ToUniversalTime(tz, true);

        DateTime utcFromStd = london.ToUniversalTime(tz, false);

        Console.WriteLine($"London: {london}");

        Console.WriteLine();

        Console.WriteLine($"UTC:    {utcFromDst} (assuming input was daylight time)");

        Console.WriteLine($"UTC:    {utcFromStd} (assuming input was standard time)");

    }

}

​

public static class Extensions

{

    public static DateTime ToUniversalTime(this DateTime dt, TimeZoneInfo tz, bool isDst)

    {

        // If we're not dealing with unspecified kind, then the normal ToUniversalTime method is used.

        // Thus, the tz and isDst parameters are ignored.

        if (dt.Kind != DateTimeKind.Unspecified)

        {

            return dt.ToUniversalTime();

        }

        // Handle invalid values (impossible values in the local time zone, due to advancing local time).

        if (tz.IsInvalidTime(dt))

        {

            throw new ArgumentException("Invalid local date and time for the specified time zone.");

        }

        // For ambiguous values, choose the offset for the indicated time zone and isDst flag.

        if (tz.IsAmbiguousTime(dt))

        {

            TimeSpan[] offsets = tz.GetAmbiguousTimeOffsets(dt);

            Array.Sort(offsets);

            TimeSpan offset = isDst ? offsets[1] : offsets[0];

            DateTimeOffset dto = new DateTimeOffset(dt, offset);

            return dto.UtcDateTime;

        }

​

        // Simple case

        return TimeZoneInfo.ConvertTimeToUtc(dt, tz);

    }

}