Creating Mongo ObjectIds manually (e.g., for testing) from a given DateTime object.

MongoDateTimeExtensions.cs

public static class DateTimeExtensions
{
	static readonly DateTime Epoch = new DateTime (1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
	public static string AsObjectIdPrefix (this DateTime dateTime)
	{
		var deltaSinceEpoch = dateTime - Epoch;
		var seconds = (int)deltaSinceEpoch.TotalSeconds;
		var secondsAsHex = seconds.ToString ("X").PadLeft (8, '0').ToLowerInvariant ();
		return secondsAsHex;
	}
}

public class DateTime_AsObjectIdPrefix
{
	[Test]
	public void Epoch_Is00000000 ()
	{
		var date = new DateTime (1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
		var expected = "00000000";

		var epochHex = date.AsObjectIdPrefix ();

		Assert.That (epochHex, Is.EqualTo (expected));
	}
	[Test]
	public void SomeDate_IsExpectedValueManuallyGrabbedFromMongo ()
	{
		//ISODate("2015-11-25T23:01:50")
		var date = new DateTime (2015, 11, 25, 23, 01, 50, DateTimeKind.Utc);
		var expected = "56563dde";

		var epochHex = date.AsObjectIdPrefix ();

		Assert.That (epochHex, Is.EqualTo (expected));
	}
}