The detention in The Philosopher's Stone was clearly set for eleven o'clock.
Your detention will take place at eleven o'clock tonight. Meet Mr Filch in the Entrance Hall.
Prof M. McGonagall
(Philosopher's Stone, Chapter 15, The Forbidden Forest).
Filch accompanies the kids down to Hagrid's hut. Hagrid complains that they're late and says he's been waiting for them.
"Abou' time," he said. "I bin waitin' fer half an hour already. All right, Harry, Hermione?"
"I shouldn't be too friendly to them, Hagrid," said Filch coldly, "they're here to be punished, after all."
"That's why yer late, is it?" said Hagrid, frowning at Filch. "Bin lecturin' them, eh? 'Snot your place ter do that. Yeh've done yer bit, I'll take over from here."
(Philosopher's Stone, Chapter 15, The Forbidden Forest).
Why did Hagrid think they were late? Had he been told the wrong time? Were they late? It seems odd that he should be hanging around for such a long time for them when the time was agreed in advance.