I left the U.S. for Canada with about 3 weeks left on my stay allowance in the U.S. I am now wondering if the 90 days kept counting down while I was in Canada? I would like to visit some family on the way back over to Europe, but have the feeling that my 90 days have now expired. I am aware that Canada does not count towards a reset in allowance. Any help appreciated!
No. The 90 days do not pause in Canada, Mexico, or nearby islands. Source (Ctrl+F, "Trips to Canada, Mexico, or nearby Islands." When you come back to the US, your leave-by date is the same actual date as it was when you first entered the US; your whole trip, from when you first enter the US until you last leave the US, must be at most 90 days unless you left North America in the middle.