Untitled

// Adapted from http://msmvps.com/blogs/jon_skeet/archive/2012/05/02/more-fun-with-datetime.aspx

using System;
using System.Diagnostics;
using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Explicit, Size = 8)]
struct DateTimeUnion
{
    [FieldOffset(0)]
    public DateTime datetime;
    [FieldOffset(0)]
    public ulong value;
}

public class DateTimesDeepDarkSecret
{
    static string DumpDate(DateTime date)
    {
        DateTimeUnion foo = new DateTimeUnion();
        foo.datetime = date;
        ulong flags = foo.value >> 62;
        ulong ticks = foo.value & 0x3FFFFFFFFFFFFFFF;
        Debug.Assert((long)ticks == date.Ticks);
        return String.Format("{{flags:{0:x}, ticks:{1:x}, datetime:{2}}}", flags, ticks, date);
    }

    public static void Main()
    {
        // Start with different UTC values around a DST transition
        var original1 = new DateTime(2012, 10, 28, 0, 20, 0, DateTimeKind.Utc);
        var original2 = new DateTime(2012, 10, 28, 1, 20, 0, DateTimeKind.Utc);

        // Convert to local time
        var local1 = original1.ToLocalTime();
        var local2 = original2.ToLocalTime();

        // Result is the same for both values. Information loss?
        var expected = new DateTime(2012, 10, 28, 1, 20, 0, DateTimeKind.Local);
        Console.WriteLine(local1 == expected); // True
        Console.WriteLine(local2 == expected); // True
        Console.WriteLine(local1 == local2);   // True

        // Convert back to UTC
        var roundTrip1 = local1.ToUniversalTime();
        var roundTrip2 = local2.ToUniversalTime();

        // Values round-trip correctly! Information has been recovered...
        Console.WriteLine(roundTrip1 == original1);  // True
        Console.WriteLine(roundTrip2 == original2);  // True
        Console.WriteLine(roundTrip1 == roundTrip2); // False

        var local3 = local1.AddYears(-1).AddDays(2);
        var local4 = local2.AddYears(-1).AddDays(2);
        Console.WriteLine(local3.ToUniversalTime().Hour); // 0
        Console.WriteLine(local4.ToUniversalTime().Hour); // 1

        // Should be a no-op?
        var local5 = DateTime.SpecifyKind(local1, local1.Kind);
        Console.WriteLine(local5.ToUniversalTime().Hour); // 1

        Console.WriteLine("original1:  {0}", DumpDate(original1));
        Console.WriteLine("original2:  {0}", DumpDate(original2));
        Console.WriteLine("local1:     {0}", DumpDate(local1));
        Console.WriteLine("local2:     {0}", DumpDate(local2));
        Console.WriteLine("expected:   {0}", DumpDate(expected));
        Console.WriteLine("roundTrip1: {0}", DumpDate(roundTrip1));
        Console.WriteLine("roundTrip2: {0}", DumpDate(roundTrip2));
        Console.WriteLine("local3:     {0}", DumpDate(local3));
        Console.WriteLine("local4:     {0}", DumpDate(local4));
        Console.WriteLine("local5:     {0}", DumpDate(local5));
    }
}