using System;

public class Program

{

    public static void Main()

    {

        var pickUpHours = 10;

        var pickUpMinutes = 30;

        var clientTimeZone = TimeZoneInfo.FindSystemTimeZoneById("Pacific/Chatham");

        var dateFormatWithTimeZone = "yyyy-MM-dd HH:mm:ss zzz";

        var expectedLocalDelivery = new DateTime(2023, 1, 18, 18, 0, 0);

        var expectedLocalPickUp = new DateTime(2023, 1, 18, pickUpHours, pickUpMinutes, 0);

        var expectedDeliveryDate = new DateTimeOffset(expectedLocalDelivery, clientTimeZone.GetUtcOffset(expectedLocalDelivery));

        var expectedPickUpDate = new DateTimeOffset(expectedLocalPickUp, clientTimeZone.GetUtcOffset(expectedLocalPickUp));

        Console.WriteLine("Expected Delivery loc: {0}", expectedDeliveryDate.ToString(dateFormatWithTimeZone));

        Console.WriteLine("Expected Delivery utc: {0}\n", expectedDeliveryDate.ToUniversalTime().ToString(dateFormatWithTimeZone));

​

        Console.WriteLine("Expected Pick Up loc:  {0}\n", expectedPickUpDate.ToString(dateFormatWithTimeZone));

        var utcTime = TimeZoneInfo.ConvertTimeBySystemTimeZoneId(expectedDeliveryDate, "Greenwich Standard Time");

        var utcTimePrepared = new DateTimeOffset(utcTime.Year, utcTime.Month, utcTime.Day, 0, 0, 0, utcTime.Offset);

        var pickUpAfter = utcTimePrepared.AddHours(pickUpHours).AddMinutes(pickUpMinutes);

        Console.WriteLine("Expected Pick Up utc:  {0}", expectedPickUpDate.ToUniversalTime().ToString(dateFormatWithTimeZone));

        Console.WriteLine("Calculated Pick Up:    {0}", pickUpAfter.ToString(dateFormatWithTimeZone));

    }

}