Look on my works, ye Mighty, and despair!

Writing tests for code that deals with time is always tricky, and Ayende’s solution is probably the most elegant around. There’s always room for some syntactic sugar though!

using System;
using NUnit.Framework;

namespace Your.Namespace.Here
{
	public class SystemTime : IDisposable
	{
		private static readonly Func<DateTime> GetDateTimeNow = () => DateTime.Now;
		private static readonly Func<DateTime> GetUtcNow = () => DateTime.UtcNow;

		public static Func<DateTime> Now = GetDateTimeNow;
		public static Func<DateTime> UtcNow = GetUtcNow;

		public static void Reset()
		{
			Now = GetDateTimeNow;
			UtcNow = GetUtcNow;
		}

		private SystemTime(DateTime now)
		{
			Now = () => now;
		}

		public void Dispose()
		{
			Reset();
		}

		public static SystemTime Is(DateTime dateTime)
		{
			return new SystemTime(dateTime);
		}
	}

	public class Examples
	{
		[Test]
		public void Change_time()
		{
			var bohemiaAdoptsTheGregorianCalendar  = new DateTime(1584, 1, 17);
			using(SystemTime.Is(bohemiaAdoptsTheGregorianCalendar))
			{
				Assert.That(SystemTime.Now(), Is.EqualTo(bohemiaAdoptsTheGregorianCalendar));
				// do something
			}
			Assert.That(SystemTime.Now(), Is.EqualTo(DateTime.Now));
		}
	}
}

And yes, I do like using dates from history :)