You are now arguing semantics and a semantic argument is not going to help the OP. One can surmise, based on the OP's original post, that her mother must leave Canada by Setp 28, whether it was printed in her passport below her entry stamp or it is the physical visa seal expiration date placed in her mothers passport. The OP was given sound advice that her mother should apply to extend her stay if she wishes to remain longer because that is the ONLY safe bet to ensure her mother does not overstay and cause herself problems at a later date. I suppose if you want to argue EXACTLY how long and how safe the OP's mother is you could ask her to respond with what date EXACTLY was stamped in your mothers passport, and what exactly did the CBSA officer tell her. Did they write anything in below the stamp such as a date she must leave by, did they orally inform her that she should leave by the expiration date, did she enter Canada right after she received her visa stamp (usually valid for 6 months from issuance) or did she wait a few months before arrival?
You see, it is all semantics, but to ensure absolute safety for the OP's mother and not finding herself in trouble at a later date, she should either leave or apply to extend her visitor visa because there is no 'implied status' conferred for just hanging out longer.