When Robert announces the armistice he says it will take effect on the morning of the 11th. At least a day passes before the household gathers to mark the end of hostilities. The armistice was not signed and announced until around 5:00 on the morning of the 11th so it would have taken effect only hours after it was announced. Also, the armistice was scheduled for 11:00 am Paris time. It would have been 10:00 in England.
When the family, staff and officers gather to commemorate the armistice, their attention is centered on the clock about to strike 11. Just before the chimes begin, this clock is shown in a close-up shot to be a tabletop clock. However, several long shots of the room, including during the sounding of the 11 chimes, show a freestanding grandfather clock against the wall behind the earl's right shoulder, which is also showing 11 o'clock and would be ostensibly chiming as well.
While the story is set around November 1918, the outdoor shots were clearly taken in summer: The trees are full of leaves, the lawns are in their brightest summer green, and the characters walk in the open without coats.