Ends at Jan 31. I think. You may still play it pass the deadline provided that you have not done any updates to the game, or you are not connected online, maybe?
Just get the season pass, the comrades dlc (you don't need ps+ unless you want to play with others, it allows an offline mode) also has story elements that would fit for chapter 14. They have stuff that can be used in the main game, plus the season pass unlocks the OP ragnanok blade and other things. Episode Ignis is the best of the three bro episodes, in my opinion.
Try fighting the Adamantoise without the ring, as what I did way back before they tweaked the ring. LOL
I don't understand the update/ availability thing, I never updated the game for ages, but when I went back to it, I thought the update downloaded automatically or was mandatory, so I'm not really certain which 'version' I was playing during Chapter 13. I've noticed now in Ch14 I can join multiple hunts (not quests) which wasn't possible upon XV's release.
So if I get the season pass I get a version of similar story elements? Because it's actually cheaper on the PSN to buy the 3 bro chapters separately.
I actually did try to fight Adamantoise a year or more ago at around L40, it was a timed quest and I never would have done it in under 20 minutes at that level. Maybe it was the last update or ch14, but it's no longer timed. When I finally level up after finishing the game I'm going to fight it without the ring.
Ah, well no idea they tweaked the ring, I'm guessing it was the update