They don’t keep count. “They” being either the Canadian or U.S. government. That is the perspective of some Canadian RVers when they are travelling into the United States. They don’t worry about overstaying in the States.
As of July 2019, phase III of the Entry/Exit Initiative was implemented and this is what it means:
CBP and the Canada Border Services Agency (CBSA) are exchanging biographic data, travel documents, and other border crossing information collected from individuals traveling between the countries at land border ports of entry. This data exchange allows both governments to expand their situational border awareness so that the record of a traveler’s entry into one country can establish a record of exit from the other country.
Importantly, a coordinated Entry/Exit information system will help the U.S. and Canada identify persons who overstay their lawful period of admission; monitor the departure of persons subject to removal orders; and verify that residency requirements are being met by applicants for continued eligibility in immigration programs.
Both governments can and will identify travellers who overstay their lawful period of admission. One of many possible outcomes could included denied entry.
We count the days and I carry the data with me. In our coach and on my smartphone.
Some of the above data came from the i94 website. This U.S. government website will report on your travel history. Enter your name, date of birth and passport information and a table with your travel history will magically appear.
Border crossing between Canada and the United States was based largely on an honour system.
Our governments are actively monitoring the movement of people when they cross the border.
Be careful with your days in the United States.
Someone is keeping count even if you are not.