I rented a movie on 23/10/15 and tried to watch it at 9pm but it wouldn't stream to my Apple TV. It now says 'expires in 1 day'. If I start watching it at 8 pm on 25/10/15 will it stop working dead on 9 pm? Or will it just let me finish?

Hello Jeewahh,

If I understand correctly, you rented a movie and want to know how long you have to finish watching the movie. I would recommend you read this article, it should be helpful in troubleshooting your issue.

About renting movies from the iTunes Store - Apple Support

You have 30 days to start watching a movie after you rent it. After you start watching the movie, you have 24 hours (in the US) or 48 hours (elsewhere) to finish it. You can watch the movie as often as you like until it expires.

If you don’t start watching a movie within 30 days, you must rent it again.

Movies you rent disappear from your iTunes library when the rental period ends.

Thank you for using Apple Support Communities.
Take care