How create unit test for logic related with time

Question

I have a use case:

    void CloseOrderWhenItIsNineAM()
    {
        if (DateTime.Now.Hour >= 9)
        {
            Order.Close();
        }
    }

How can I create unit test for this?

Answer 1

Refactor the method to this:

void CloseOrderWhenItIsNineAM(DateTime now)
{
    if (now.Hour >= 9)
    {
        Order.Close();
    }
}

Then pass DateTime.Now to it when using it normally, and unit test it by passing whatever makes sense.