I think Binary said on Tama-Zone that it's based off a folktale. If I remember correctly, he said that in the story a man goes and visits some fairy people. When he leaves, they give him the Tamatebako but tell him never to open it. When he returns home, he can't find anyone that he recognises. In desperation, he opens the Tamatebako and transforms into an old man. The Tamatebako had held the time that had passed while he was away.