I just made some progress on "The Hungry Deserter" quest in Ostagar and the quest text advises me to continue in the evening:
The chest will very likely only be reachable when the Tranquil are not present, such as when they're gone for the evening.
It's still nice and sunny out, so is there a way for me to just skip ahead a few hours until darkness similar to the wait mechanic in Fallout?
Dragon Age: Origins has no passage of time like Fallout. Pretty much the entire game takes place the daytime. Instead, evening or nightfall is triggered by certain events. Ostagar is home to one of these events.
It only becomes evening in Ostagar once you finish a couple of your Grey Warden quests in the Korcari wilds (Tainted Blood and The Grey Wardens' Cache). That's what your quest text is talking about.
Warning: Get the chest whilst you can. If you trigger other events to make the story progress further, the chest will no longer be available, though the game will mark the quest as complete for you.
