A rented movie has a fixed expiry date while a bought movie does not.
You can watch a rented movie anytime for a specific rental duration (usually 30 days after purchase but can vary from case to case basis).
Please note that once you start the playback, the rented movie will expire after a specific duration (usually 48 hrs after playback but can vary from case to case basis). You can find the details about the rental duration of a movie on the respective movie details page.