This one is more specialized, especially since it is dynamic in nature.
Use this one for general purpose night scenes especially if you are planning on using another sky dome on top of it, like one of the excellent ++skies sky domes.Specifically, in this article, we will discuss only the Time of Night sky box as that is what our questioner wanted to know about but in the interest of inclusiveness, we will briefly explain the differences, in the broadest terms, of when to use which one. However, the two have rather different applications and it really helps to choose the right one for your scene. In fact, RAY-MMD comes also with the SKY NIGHT sky box for rendering night time scenes.
Someone wrote asking about how to use the Time of Night sky box that comes as standard with RAY-MMD so this is what this article will be about.