A well designed unit test example where the current day is hidden behind an abstraction. From: http://hjerpbakk.com/blog/2013/9/24/the-little-test-that-could-not.html

GoodTest.cs

[TestFixture]
public class GoodUnitTestExample {
    [Test]
    public void AgeMustReturnCorrectAge() {
        IClock clock = new StubClock(new DateTime(2013, 9, 2));

        var aPerson = new PersonGood(clock) { TimeOfBirth = new DateTime(1983, 9, 8) };

        Assert.AreEqual(29, aPerson.Age);
    }
}

public class PersonGood {
    private readonly IClock clock;

    public PersonGood(IClock clock = null) {
        this.clock = clock ?? new Clock();
    }

    public DateTime TimeOfBirth { get; set; }

    public int Age { 
        get {
            var today = clock.Today;
            int age = today.Year - TimeOfBirth.Year;
            if (TimeOfBirth > today.AddYears(-age)) --age;
            return age;
        }
    }
}

public interface IClock {
    DateTime Today { get; }
}

public class Clock : IClock {
    public DateTime Today { get { return DateTime.Today; } }
}

public class StubClock : IClock {
    private readonly DateTime now;

    public StubClock(DateTime now) {
        this.now = now;
    }

    public DateTime Today {
        get {
            return now.Date;
        }
    }
}