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.”
Who’s watching TV?
Do you know who’s going to speak?
Who’s ready to go?
Who’s in the kitchen?
Who’s already eaten?
Whose is the possessive of “who” or, somewhat controversially, “which.”
Whose book is this?
Do you know whose car this is?
I know a woman whose kids study there.
Whose side are you on?
An idea whose time has come.
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.