Who knows the difference between who’s and whose? Here’s a lesson whose time has come.
Who’s is a contraction of “who is” or, less commonly, “who has.”
Whose is the possessive of “who” or, somewhat controversially, “which.”
The Bottom Line
The trouble here is due to the apostrophe, which on 99% of English words indicates possession, but on this one simply indicates a contraction. If you can replace the word with who is or who has, use who’s. If not, use whose.