Is there a missing hour?

Quote	Source
Snape strode forward, past Dumbledore, pulling up the left sleeve of his robes as he went. He stuck out his forearm and showed it to Fudge, who recoiled. "There," said Snape harshly. "There. The Dark Mark. It is not as clear as it was an hour or so ago, when it burned black, but you can still see it." ... "Severus," said Dumbledore, turning to Snape, "you know what I must ask you to do. If you are ready ... if you are prepared ..." "I am," said Snape. ... "Then good luck," said Dumbledore, and he watched with a trace of apprehension on his face, as Snape swept wordlessly after Sirius."	Book 4, Pages 709-710,713 Goblet of Fire, Chapter 36, "The Parting of the Ways"
"But you didn't return when he came back, you didn't fly back to him at once when you felt the Dark Mark burn -" "Correct. I returned two hours later. I returned on Dumbledore's orders." "On Dumbledore's -?" she began, in tones of outrage. "Think!" said Snape, impatient again. "Think!" By waiting two hours, just two hours, I ensured that I could remain at Hogwarts as a spy!"	Book 6, Page 28 Half-Blood Prince, Chapter 2, "Spinner's End"