The observation by HamiltonApplicant is a well-known, well-understood, and commonly given suggestion or recommendation, for good and rather obvious reasons.
Any circumstance which corroborates residency ties in Canada tends to corroborate presence. The more the whole picture reflects a person whose life is well-settled and centralized in Canada, the more comfortable a total stranger bureaucrat is likely to be that the applicant's account of days in Canada does not need closer examination.
In contrast, any circumstance which indicates ties abroad or the possibility the applicant is not fully settled in Canada . . . such as one spouse applying when the other does not, suggesting the couple has not fully settled together in Canada . . . can lead to questions, can point to what CIC and IRCC have, at various times, referred to as risk indicators or reasons-to-question-residency (or question presence).
As for "the number of days in Canada," in particular, in the routine application the only direct evidence of the number of days actually spent in Canada is the applicant's signature on the presence calculator attesting (claiming) that all dates of entry into Canada and dates of exit from Canada are accurately listed. This calculation is thus dependent on an inference that the applicant remained in Canada on the days between the last reported date of entry and the next reported date of exit. All it takes is some doubt in a total stranger bureaucrats thinking to put that inference at risk.
After all, there is no requirement at all that a PR have been employed while in Canada, but not only must the applicant report all work history during the relevant time period, there must be NO gaps in that work history (thus, for example, periods of unemployment must be specifically reported). Full employment at a job in Canada corroborates (reinforces) the proposition the applicant was present in Canada during that time. And that is the biggest reason work history is required. (And the converse: any indication of employment abroad will raise questions about time spent in Canada.)
Risk indicators do not mean an application will fail. So it is not as if spouses applying separately will in itself cause a problem. But the vast majority of applicants prefer to avoid non-routine processing, and especially prefer to avoid RQ. Thus, most applicants prudently pay attention to more than just their accounting of the number of days when deciding the right time to apply. Waiting a little longer to apply together with a spouse is simply the more prudent approach since the respective residency and presence information will, as HamiltonApplicant has said, reinforce evidence of living a life together in Canada, thus reinforcing the declaration of days present in Canada.
If, however, one spouse has indeed spent many more days abroad, and especially if the spouse has continued to have employment abroad, or has other circumstances which might invite non-routine processing (that is, has other risk indicators), separate applications make sense. No reason bogging both applications down because one gets non-routine processing and elevated scrutiny.
In the presence calculator, in work history, and in address history, the applicant needs to cover the full period of time in the relevant time period. When the 3/5 and pre-PR credit rules take effect, this will probably be the last five years or since coming to Canada, whichever was more recent. (We need to wait to see actual language in form and presence calculator and instructions to know for sure how IRCC will describe the relevant time period.) NO gaps. And no gaps means no gaps.
How to describe, in the application, such as in the work history section, a settling-in period of time prior to starting a job or starting school, is a personal decision, recognizing again that the applicant should not leave any gaps in time, and otherwise the applicant needs to give his or her best, HONEST answer. "Settling-in" should work for some, "looking for work" might work from someone else, "waiting for xxx to start" might work, and for some simply "unemployed" or "between university sessions," might work. As long as there is no gap, what is stated is an honest answer and is otherwise consistent (a nine month settling-in period, in contrast, could raise questions), the details will ordinarily not be important.
No need to overthink things.
Likewise the tax filing obligation section. No need to overthink things. Here again, prospective applicants need to wait and see what the actual form and instructions say. And follow the instructions. But sure, like the current form it will probably ask for information about the preceding tax YEARS (six currently, five after the law changes). For an application filed in 2017, obviously (for most) the most recent TAX YEAR is 2016.
