When nanny West is about to be sacked, Cora rings downstairs for someone to come up. The bell rings all the way in the basement, as we've seen many times. Then the person has to climb at least 3 flights of stairs and walk down a long hallway to get to the room. Yet Mrs Hughes shows up exactly 14 seconds after the bell is rung. This would not have been possible even if they had an elevator, which they did not. It would have taken a minimum of 90 seconds to get from the basement to the nursery.
The prior events culminating in Matthew's death took place in September 1921. It is mentioned that the current events take place 6 months after Matthew's death. A character mentions it being St. Valentines day. In 1921 as in modern times St. Valentines day was celebrated on February 14. As such, it would only be 5 months or less since Matthew's death.