Days as visitor:
As others have noted, days actually physically present in Canada, during the five year eligibility period prior to the date the application is made, COUNT.
As the OP apprehends, however, the absence of formal documentation evidencing dates of entry into Canada can be an issue.
Many times an American who comes into Canada to be with a partner will be issued a Visitor's Record (as I was), which will document status and date of entry.
With no formal documentation, no status to work so presumably no work history in Canada for that time period, no school records for that time period, no record of health care in Canada, and so on, sufficiently proving actual presence in Canada during those periods can be a bit problematic. The burden of proof is on the applicant.
Generally an American citizen should have no need to rush becoming a Canadian citizen, so it may be a good idea to wait, to wait to have enough days without relying on undocumented days as a visitor, before applying. (While I had additional reasons for waiting longer, and even though I had a VR which could document a large period of time, this was a factor in my decision to wait longer to apply.)
This is a situation in which an oft repeated caution applies:
rushing the application can mean it takes longer to become a citizen.
And even if you ordered the ENTRY records from the US, they would most likely not list your entries since the US doesn't collect entry dates for US nationals.
I am not sure of the precise policy, but I believe the U.S. does collect entry data for U.S. citizens. That information, however, may not be included in data shared with Canada (so Canada may not have direct access to it).
In particular, I do not stay current regarding the U.S. rules, policies, or practices, not closely anyway. So I am not sure of the formal policy.
I can say, however, the U.S. provided me a list of my entries into the U.S. (via FOIA request). And I am a U.S. citizen (with a strong preference for emphasizing my Canadian citizenship). The records went back ten years, prior to when even the U.S. started examining passports (or other acceptable documents) every time (or almost every time anyway), so of course it was incomplete relative to older border-crossings, but appeared to be at least very close to complete since around 2005 or 2006 or so. I requested this information well before I applied for citizenship, mostly because I was curious (I kept an ongoing log of all travel outside Canada, which is what every PR should do, so it was not that I needed it) and actually became a citizen before I finally received it. I believe they provide faster service these days.
