Van Zan was talking about a specific time of day during sunrise and sunset called "magic hour" or "golden hour", when the light from the sun is softer. Van Zan had figured out that the dragons have impaired vision during that time of the day. We see evidence of this during the final battle with the male dragon in London when we see things from its perspective, its vision is distorted, somewhat blurred and unfocused, making it harder for the dragon to see clearly.
