After downloading and installing free playstation plus games, when I was checking the information about these games (by pressing the "triangle" button) some had expiration dates (they expire when my membership ends) and some games don't have expiration dates at all (and no "starting date" either).

Does that mean that those games do not expire when the membership ends ?

Weird. I just checked and it's the same with some of mine. Also, most of my PS1 games show as expired (expired at my old subscription expiration date) but they work, probably because my PS+ is still active. I'll check what still works and what doesn't when my account expires in three days.

Any games you buy off PS+ that have a discount, you'll be able to access at any time, even if your sub runs out. Anything you get "free", you lose access to once your sub runs out.

EDIT: I just re-read the OP. Does this only happen on the "free" games? Because if that's the case, then I have no idea why that would happen.

@two_socks: When I checked Bloodrayne Betrayal, Rock of Ages and Pacman CE DX didn't have expiration dates. Those were all free.

@CharleyTony: My PS+ Account has just expired. It seems that the games with no expiration date were games that had you download the demo and a full game unlock code. When you start them up the demo launches and not the full game. You can't use your save files and all your PS+ related content is removed from the download list so you can't redownload it.

It's because of what they are saying that they are only free for "x" time when your sub runs out you lose it. However, in your case what happens if you keep your sub is when the time runs out it does a spot check to see if you still have an account if you do "boom" you get it for "x" time again without losing it. So technically if you run out of time you still have your game until the "soft check"