class Person
{
    public DateTime DateOfBirth { get; private set; }

    public DateTimeSpan Age => DateTime.Now - DateOfBirth;
}

[Fact]
public void TestAge()
{
    var mockClock = new Mock<ISystemClock>(); 
    mockClock.Setup(m => m.GetCurrentDateTime()).Returns(new DateTime(2019, 1, 1));
    var person = new Person(mockClock.Object);
    person.DateOfBirth = new DateTime(2000, 1, 1);
    person.Age.Should().Be(19);
}

public interface ISystemClock {
    DateTime GetCurrentDateTime();
}

public class SystemClock: ISystemClock {
    public DateTime GetCurrentDateTime() => DateTime.Now;
}

containerBuilder.RegisterType<SystemClock>().As<ISystemClock>().SingleInstance();