Testing the behavior of the LaunchRockets method

How to write a unit test for the LaunchRockets method, allowing changes to the code structure: adding, removing, or modifying parts of it.

public class Defender
{
    private RocketService rocketService;

    public Defender()
    {
        rocketService = new RocketService();
    }

    public void Start()
    {
        rocketService.LaunchRockets();
    }
}

public class RocketService
{
    public void LaunchRockets()
    {
        if (DateTime.Now.Hour > 10)
        {
            // implementation of launching rockets
        }
    }
}