I've recently started using this katana and I've found out that the light damage additional effect doesn't go off during the day. It seems it can only go off at night, between 18:00-6:00 game time, same as the latent conditions of some pieces of NIN AF.
According to the definition in wikipedia.org, the word 'shiranui' means 'phosphorescent light'. That means it absorbs light during the day and releases it during the night?