How greatly may the actual arrival time at the Canadian land border differ from the ArriveCAN form's declared time?

I plan to enter Canada from the US by car on a certain day, via the Douglas Border Crossing. I must complete the ArriveCAN form, which asks me to declare not just the date, but the precise time of arrival, to the specific minute.

But I don't know when exactly I will arrive. Lots of things can make me arrive a few hours earlier or later when I drive myself to the land border: chatting after dinner, traffic jams, border line-ups, and more. I am very confident about my 12-hour period of arrival. I have moderate confidence about a 1-hour window of arrival. I have no confidence about the exactly minute when I will arrive.

How greatly may the actual arrival time at the Canadian land border differ from the ArriveCAN form's declared time? What accuracy limits are published Canadian government policy? What about our experiences? What differences have people had success with — or problems? Is the answer different for different categories of travellers, e.g. Canadians returning home after less than 72 hours in the US, vs US or other citizens visiting Canada?

(This question applies to the rules for entering Canada in effect from November 30, 2021, and onwards. As long as those rules are substantially unchanged, this question and its answers remain valid. But because of the pandemic, the rules will probably change, and that may require a different question and different answers.)