Midnight
In short: The hour when the enchantment expires in Perrault’s Cinderella.
Motif
How does it work in the tale?
The reader learns the deadline with the heroine. The ball then becomes a scene about when she must leave as well as whom she might meet.
How does the same hour change two balls?
In Perrault, Cinderella leaves the first ball in time. At the second she forgets the hour and runs as midnight begins to strike. Readers already know the spell’s limit, so the chimes sound like a warning. What changes on the second night is when Cinderella notices the time.