Some of the canon behind this will depend on what the official Dark Sun books suggests (adjusted to taste, of course), but there's lots of room for this to fit whatever goals the characters are gear towards. I was certainly inspired to this by the Dark Sun game I played in PAX East (where they announced the fall of Kalak basically mid-game).
It could be fun for the game to feel like it's unfolding in "Real Time", to some extent. I don't think I want IRL time = game time, but there would be an emphasis in making the most of every hour of time, and packing each day with conflict. (Not always combat, but the players may say "We spend the rest of the afternoon plotting with Nobles and finding the Mole", and that becomes an appropriate Skill Challenge.)
Further mechanic alterations: with a high enough pace of encounters (combat + skill + general RP / quest XP), I could see characters leveling every day or so of game time. I might offer other methods of restoring a few Healing Surges / Dailies, depending on how things go, but the resource crunch in a crisis time would be an interesting motivator.
