Use Assert.That and Is.Equal constraint instead of Assert.AreEqual. Below is a code sample from the Nunit website itself
DateTime now = DateTime.Now;
DateTime later = now + TimeSpan.FromHours(1.0);
Assert.That(now, Is.EqualTo(now) );
Assert.That(later, Is.EqualTo(now).Within(TimeSpan.FromHours(3.0)));
Assert.That(later, Is.EqualTo(now).Within(3).Hours);