How to generate guid from DateTime?

Question

I want to generate a Guid v.1 from a DateTime. I try to use the class described here:

https://gist.github.com/nberardi/3759706

But after getting a DateTime from a Guid, I tried to get the same Guid as the original using that DateTime, and it gave me a different Guid. Here's the code I used:

  string uuidString = "9e5713bb-bb4c-11e8-9d6c-12345678df23";
  Guid gui = new Guid(uuidString);
  DateTime dateTimeFromGuid = GuidGenerator.GetDateTime(gui);
  Console.WriteLine(dateTimeFromGuid.ToString("dd/MM/yyyy hh:mm:ss"));
  Guid guidFromDateTime = GuidGenerator.GenerateTimeBasedGuid(dateTimeFromGuid);
  Console.WriteLine(guidFromDateTime); //7909dbbb-bb33-11e8-9f6a-95d8e90ccf10

How can I get the original GUID back from a DateTime?

Answer 1

If you want something simple that can be converted back and forth regardless of what machine you are on. Then, you can convert the date to a byte array and use that to create the Guid. This is non-standard, but will generate the same exact Guid for each DateTime.

var today = new DateTime(2018, 9, 18, 10, 59, 00);

var bytes = BitConverter.GetBytes(today.Ticks);

Array.Resize(ref bytes, 16);

var guid = new Guid(bytes);

Console.WriteLine(guid); //bd02b200-1d55-08d6-0000-000000000000

And back to date.

var dateBytes = guid.ToByteArray();

Array.Resize(ref dateBytes, 8);

var date = new DateTime(BitConverter.ToInt64(dateBytes));

Console.WriteLine(date); //9/18/2018 10:59:00

If you want to use extensions then.

class Program
{
    static void Main(string[] args)
    {
        var date = new DateTime(2018, 09, 18, 12, 00, 00);

        var guid = date.ToGuid();

        Console.WriteLine(guid); // 428a6000-1d5e-08d6-0000-000000000000

        var back2date = guid.ToDateTime();

        Console.WriteLine(back2date); // 9/18/2018 12:00:00
    }
}

public static class DateTimeExtensions
{
    public static Guid ToGuid(this DateTime dt)
    {
        var bytes = BitConverter.GetBytes(dt.Ticks);

        Array.Resize(ref bytes, 16);

        return new Guid(bytes);
    }
}

public static class GuidExtensions
{
    public static DateTime ToDateTime(this Guid guid)
    {
        var bytes = guid.ToByteArray();

        Array.Resize(ref bytes, 8);

        return new DateTime(BitConverter.ToInt64(bytes));
    }
}