using System;

using System.Diagnostics;

using System.Linq;

using System.Runtime.InteropServices;

public class Program

{

    public static void Main()

    {

        // Note, the following line is just an example for clarity.  If you know which form of identifier your platform supports

        // or are using TimeZoneConverter, or are using .NET 6+ (which will have conversions built-in), then you can just use the appropriate simple string.

        string tzid = RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? "GMT Standard Time" : "Europe/London";

        // Get the time zone and show a few different days and their duration.

        TimeZoneInfo zone = TimeZoneInfo.FindSystemTimeZoneById(tzid);

        ShowDayDuration(new DateTime(2021, 3, 28), zone);

        ShowDayDuration(new DateTime(2021, 6, 19), zone);

        ShowDayDuration(new DateTime(2021, 10, 31), zone);

    }

    public static void ShowDayDuration(DateTime date, TimeZoneInfo zone)

    {

        TimeSpan duration = GetDayDuration(date, zone);

        Console.WriteLine($"Duration of {date:yyyy-MM-dd} in zone {zone.Id}: {duration.TotalHours} hours");

    }

    public static TimeSpan GetDayDuration(DateTime date, TimeZoneInfo zone)

    {

        // Just to make sure the input value is strictly a date with the time truncated.

        Debug.Assert(date.Kind == DateTimeKind.Unspecified);

        Debug.Assert(date.TimeOfDay == TimeSpan.Zero);

        // Subtract the start of the given date and the subsequent date.

        DateTimeOffset dto1 = GetStartOfDay(date, zone);

        DateTimeOffset dto2 = GetStartOfDay(date.AddDays(1), zone);

        return dto2 - dto1;

    }

    public static DateTimeOffset GetStartOfDay(DateTime date, TimeZoneInfo zone)

    {

        // Just to make sure the input value is strictly a date with the time truncated.

        Debug.Assert(date.Kind == DateTimeKind.Unspecified);

        Debug.Assert(date.TimeOfDay == TimeSpan.Zero);

        TimeSpan offset;        

        if (zone.IsAmbiguousTime(date))

        {

            // When the date/time is ambiguous, use the larger of the two offsets because the resulting value will come first sequentially.

            offset = zone.GetAmbiguousTimeOffsets(date).Max();

        }

        else

        {

            // When the date/time is invalid, find the next valid time.  Search by 15 minute increments to accomodate oddities of various time zones.

            while (zone.IsInvalidTime(date))

            {

                date = date.AddMinutes(15);

            }

​

            offset = zone.GetUtcOffset(date);

        }

        return new DateTimeOffset(date, offset);

    }

}