using System;

using System.Globalization;

​

public class Program

{

    public static void Main()

    {

        // Use a specific time zone

        TimeZoneInfo timeZone = TimeZoneInfo.FindSystemTimeZoneById("America/Los_Angeles");

        // Start with a DateTimeOffset to prevent ambiguity

        DateTimeOffset startDateTimeOffset = DateTimeOffset.Parse("2021-03-14T18:00:00-08:00", CultureInfo.InvariantCulture);

        // Define the duration of the shift

        TimeSpan shiftLength = TimeSpan.FromHours(8);

        // Add the duration and convert to the same time zone to adjust to the correct offset

        DateTimeOffset endDateTimeOffset = TimeZoneInfo.ConvertTime(startDateTimeOffset + shiftLength, timeZone);

        // The result will be correctly adjusted for DST

        Console.WriteLine(endDateTimeOffset.ToString("yyyy-MM-dd'T'HH:mm:sszzz", CultureInfo.InvariantCulture));

        // Output: "2021-03-15T03:00:00-07:00"

    }

}