In addition to other responses:
Accurately and completely reporting all dates that the applicant exited Canada and entered Canada is one of the most important things to do right when making an application for citizenship.
While there is some leeway for approximations or small errors, that leeway probably continues to be rather narrow. IRCC is perhaps a bit more flexible than CIC was during the Harper years, but there really is rather little wiggle-room when accounting for travel abroad. Including brief travel to the U.S.
In particular, IRCC has little reason to be sympathetic to claims about how difficult it is provide such detailed information. The PR was there, in person, each and every time he or she left Canada, each and every time he or she entered Canada, and indeed there is no one else in the world who is in a better position to know and report each and every trip abroad than the person who made those trips.
In other words, the importance of getting this information as complete and accurate as possible cannot be overstated. Again, this is true even though the instructions allow an applicant to provide, with explanation, some approximate dates. Approximations are almost certain to elevate the risk of non-routine processing significantly if not dramatically. And any significant omissions will almost certainly cause delays and problems.
How well a PR can reconstruct this information if he failed to keep good records depends on that PR's particular circumstances, especially as to those related to his travel, such as whether his credit card records or employment and business records, might help the individual to reconstruct a near complete account of travel.
Bottom-line, for the PR who failed to keep regular records of travel abroad, that PR needs to make a serious, diligent, dedicated, extensive effort to track down each and every trip, before even thinking of sending off an application.
A return to Canada the same day one exited Canada, or the very next day, will net zero days abroad.
Beyond that, this question has to be about travel to the U.S. unless it is not about trucking or such but about employment with an airline or such as that.
Foremost, IRCC now requires that all trips abroad be declared. A half hour jaunt across the Queenston Heights bridge and back, to buy cheap gas at Smokin Joe's in Lewiston for example, needs to be declared. It nets zero days abroad, but it still needs to be declared.
Make sure to get the calendar date right. Stuck in traffic on the Ambassador Bridge one evening when headed back to Windsor from Michigan, so you do not actually get through customs/PoE before midnight, that is a return to Canada the next day.
For any Trans-Pacific flights be sure to not base the reported exit from Canada date on the entry stamp in the destination country . . . depending on the particular flight, the exit from Canada date will be at least one or two days earlier.
In any event, it is not the number of hours one has traveled to the U.S., but rather simply the date that Canada was exited and the date of re-entry into Canada. Put those into the calculator accurately, the calculator will accurately give a count of days outside Canada.
By the way, it makes no difference at all whether the trip was personal or business, for recreation or on behalf of a Canadian employer.
