The expiration date you are referring to indicates the last date on which you are able to "land" in Canada to become a PR. I believe it is usually 12 months from the date you did your medicals, or the date on which your passport expires (whichever comes first). What you have to be concerned about is whether you meet your residency obligations, and are able to prove it, at the time you present yourself to a border official at the point of entry.
