SystemTime.cs

public static class SystemTime
{
	private static readonly ThreadLocal<Func<DateTime>> _Now = new ThreadLocal<Func<DateTime>>(() => () => DateTime.Now);

	public static DateTime Now
	{
		get { return _Now.Value(); }
	}

	public static ResetSystemTime Set(DateTime executionTime)
	{
		_Now.Value = () => executionTime;
		return new ResetSystemTime();
	}

	public static void Reset()
	{
		_Now.Value = () => DateTime.Now;
	}

	public class ResetSystemTime : IDisposable
	{
		public void Dispose()
		{
			Reset();
		}
	}
}

public class Bar
{
	public DateTime LastModified { get; set; }

	public void UpdateTime()
	{
		LastModified = SystemTime.Now;
	}
}

[TestFixture]
public class Tests : AssertionHelper
{
	[Test]
	public void SetsLastModifiedTimeToCurrentTime()
	{
		var foo = new Bar();
		var executionTime = new DateTime(2014, 1, 1);

		using (SystemTime.Set(executionTime))
		{
			foo.UpdateTime();
		}

		Expect(foo.LastModified, Is.EqualTo(executionTime));
	}
}