The expiration date for all free PS+ content, is always the same as the expiration date for your PS+ subscription. When you renew your subscription, it auto-updates the free content expiration date.
*Edit* Have you run Darksiders since you last added time to your PS+ subscription? In the past, I had to run each title individually to re-sync with my PS+ subscription, but the last time I added time it resynced all of my content automatically, without needing to run them.
Content does not expire when redeemed. If you download a free game it will appear on your download list and even a year later when you renew your Plus subscription you will be able to play it again. The conditions are you need to have it as a download otherwise the deal is gone and you have a current Plus subscription.