It seems I can find the answer:
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8. I have been a permanent resident (PR) for less than five years. How will my five-year period be calculated?
If it has been less than five years since you became a permanent resident of Canada, the five-year period will be calculated from the date of your landing, and the officer will consider whether it is possible that you could accumulate 730 days before the end of that five-year period.
For example, if you became a PR on 01 June 2003, spent sixty days in Canada, and wish to return to Canada on 01 February 2004, you will meet the residency criterion: your five-year period ends on 31 May 2008. You have already accumulated 60 days in Canada, and between 01 February 2004 and 31 May 2008, you will be able to accumulate the 670 days you require. On the other hand, if you became a PR on 01 June 2000, spent 30 days in Canada, and now wish to return, you will not meet the criterion: you still require 700 days, and it is not possible for you to acquire these in the time remaining before your five-year period expires on 31 May 2005.
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Source:
http://www.canadainternational.gc.ca/uae-eau/visas/permanent_resident.aspx?lang=eng#n2