In the chapter of Robert Louis Stevenson's novel The Strange Case of Dr. Jekyll and Mr. Hyde titled "Dr. Lanyon's Narrative," Lanyon describes the manner in which he receives a letter from his old friend Dr. Henry Jekyll. The full content of that letter is then displayed for the benefit of the reader. The letter requests of Lanyon that he visit Jekyll's home and, with the assistance of Jekyll's butler, open a locked cabinet, retrieve its contents, and return to his, Lanyon's, home in Cavendish Square. This unusual request is then followed by an additional request that Dr. Lanyon be home at the specified time for the purpose of receiving another visitor. The passage from Jekyll's letter reads as follows:
"At midnight, then, I have to ask you to be alone in your consulting room, to admit with your own hand into the house a man who will present himself in my name, and to place in his hands the drawer that you will have brought with you from my cabinet."
Sure enough, at precisely midnight ("Twelve o'clock had scarce rung out over London, ere the knocker sounded very gently on the door"), Dr. Lanyon is disturbed by the arrival of a short, oddly-dressed man. The man's clothes are not in-and-of themselves odd; rather it is the extent to which the man is clearly too short for this otherwise well-made apparel. The visitor is, of course, Mr. Hyde, who proceeds to mix the concoction that Lanyon had retrieved from Dr. Jekyll's cabinet and drinks it, resulting in a violent, convulsive reaction. As Dr. Lanyon describes this transformation occurring before his eyes, he exclaims:
". . .there before my eyes pale and shaken, and half fainting, and groping before him with his hands, like a man restored from death there stood Henry Jekyll!"
So, at midnight, Hyde appeared at Dr. Lanyon's door, proceeded to consume the chemical mixture that Lanyon had retrieved from Dr. Jekyll's home, and was transformed into Jekyll. The answer to the student's question, then, is midnight.