Depends on the specific law and what the purpose is, they can share things like manifests under the various anti-terrorism laws so they will at least have your name and the date you entered the US on both sides of the border. Date you came back to Canada though going through a land border? Hmm, I don't think they do. Not yet anyway. But from what I've read about the WHTI I think they will start keeping track next year.
The Border Patrol have access to the USCIS entry records too, so if you get caught in a spotcheck in the US (which can happen e.g. on ferries near the border or roads near the border) they will know how long you have been in the country.




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