I am assuming that if my physical present days are way more than 1095, one or two days mis calculation should be alright, but not sure so that's why I am asking.
There are two different issues at stake:
-- whether an error in what the applicant reported indicates the applicant fell short of the 1095 days presence requirement (which if so, if short even by a day, the application must fail), or
-- whether there is cause for IRCC to have broader questions and concerns about the accuracy of the applicant's information, which could be triggered by an error in what the applicant reported
For the first of these, above, a margin bigger than the mistake protects the application from being denied.
If in contrast IRCC sees reason to question the applicant's information more thoroughly, especially if there are credibility concerns, a big margin may help some but how aggressively IRCC examines the applicant will depend mostly on why IRCC is concerned and how readily those concerns are resolved. This is a more complicated issue.
@akbardxb identifies an important distinction, the difference between a "
miscalculation" versus errors or omissions in reporting travel history. If the applicant uses the online presence calculator, which an applicant for sure should, there will not be any "
miscalculation" so long as the applicant accurately and completely reports ALL travel history details. There should be zero chance of a miscalculation as long as the applicant accurately and completely lists every date of entry and date of exit, thus identifying each and every trip across the border. The online calculator automatically counts and totals days in Canada.
Thus, if there is a "
miscalculation," what that really involves is some discrepancy in travel history, either an error or omission in reporting dates of entry and exit.
IRCC recognizes people make mistakes, and generally does not penalize applicants for making incidental or, as some frame it, "
innocent" mistakes. If, however, an innocent mistake changes the total number of days present in Canada by enough it means the applicant fell short of the 1095 day requirement, IRCC cannot grant citizenship. This is not penalizing the applicant for the mistake. This is enforcing a requirement mandated by law, denying the application because the applicant fails to meet the necessary requirements.
There have been many credible anecdotal reports from forum participants with tales of woe, the application on its way to being rejected (so best course is to withdraw sooner rather than later), when they applied with very little margin and it was later determined they made a mistake resulting in their having fewer than the required 1095 days.
In contrast, there have been numerous credible anecdotal reports by forum participants similar to the one where a PR reported they learned during the interview they had failed to include a three week trip abroad in their travel history, but they had applied with a big margin, the processing agent was satisfied they had made an innocent mistake, and they proceeded to be scheduled for the oath, no problem, no delay, no RQ.
The latter, the mistake in failing to report a three week trip but having a big margin, goes to precisely what you are asking. Will a big margin, big enough to cover mistakes, mean things will still be OK? In the example I just referenced, it was OK. But that's only half the story.
Leading to the anecdotal experience reported by
@scylla, where an applicant who failed to report a single day trip to the U.S. ended up with RQ (and all the hassle, delay, and profoundly intrusive questions that involves) despite having a big margin.
Generally applicants are never told why they got RQ. Many times the RQ'd applicant can figure it out. If they missed reporting a three week trip and got RQ, it is quite likely that omission triggered the RQ (even though another applicant who missed reporting a three week trip did not get RQ). Still, that's largely speculation. Usually there is no way to know for sure why IRCC imposed RQ.
In contrast, usually it would be highly unlikely that a failure to report a day trip, even a number of day trips (let alone just one), would trigger RQ. There are scores and scores of anecdotal reports of cases where errors on that scale, and bigger errors, did not trigger RQ.
But as
@scylla reports, an applicant with a big margin over the minimum, and whose only mistake in travel history was omitting a single day trip, can run into RQ. I will not try explaining why that happened in that case, but generally this would signal there were other reasons why IRCC had concerns and pursued RQ. Indeed, generally it would be other factors that triggered RQ rather than the applicant's mistake leaving out a single day trip.
I will address one particular aspect of omitting a day trip: so far, at least until very recently, IRCC compared the applicant's travel history with CBSA history of ENTRY dates, not exit dates (even though for most applicants IRCC could get exit dates from CBSA, and if things go to a more formal investigatory procedure it will). So, if the applicant fails to report a day trip, what IRCC sees is an entry date without the corresponding exit date prior to that entry. Generally this would merely invite an interview question and if the applicant readily and credibly explains,
oh, must have been the time I drove into Niagara Falls, N.Y., to do some U.S. shopping and fill my gas tank, and came back to Canada the same day, NO PROBLEM. But if the total stranger bureaucrat is not persuaded that is an honest explanation, that can lead to concerns about the overall credibility of the applicant and serious doubts about the veracity of the travel history provided by the applicant. A big margin will not answer those kinds of questions, and indeed in an ironic sort of way may actually aggravate the agent's level of skepticism.
If and when IRCC has doubts about the applicant's travel history, that expands the inquiry well beyond travel history dates. It warrants remembering that calculating physical presence based on travel history depends on an inference of presence in Canada between the date of a known entry and the next reported date of exit. Once the travel history is in question, that inference might carry little weight, and the assessment of presence can depend on proof of actual presence for the periods of time in-between, proof showing presence in Canada the days following a known entry.
Thus, for example, if an applicant has reported entering Canada January 3, 2020 and not leaving Canada until December 19, 2021, and thus is relying on credit for 717 days during that period of time, if IRCC has doubts about the applicant's reported travel history, the applicant may need to prove actual presence in Canada for that nearly two year period of time. And yeah, that can mean providing documentation showing where the applicant was during all those months and weeks, and absent such proof, that can eat up a big margin.
There seems to be a tendency among many to entirely rely on travel history doing the job. That only flies if IRCC accepts the applicant's travel history. If IRCC doubts the travel history, proof of physical presence is about actually proving where the applicant was, potentially day-to-day.
To be clear, the calculation of presence based on travel history, inferring presence in Canada all the days in-between a known date of entry and the next reported date of exit, which is how the online presence calculator works, does fly for the vast majority of applicants. IRCC generally trusts and relies on the travel history for the vast majority of applicants. But if and when IRCC no longer trusts and thus does not rely on the travel history, it's about counting days otherwise shown to have been actually spent in Canada.
All of which illustrates how important it is to get the travel history as complete and accurate as possible. A good margin will make up for most minor mistakes, but if discrepancies in the dates reported trigger significant doubts about the applicant's credibility, that could spell a long haul in getting through the process and taking the oath.