I know the OP was about being able to leave a fort for a day, but it's also interesting to ask what it would take to create a truly Eternal Fort.
After the first generation die, none of the dwarves will have any labors allocated other than hauling. And there's no eternal source of food that doesn't require a labor allocation or some player input - birds will eventually die and swapping between breeding and egg-collecting requires player input. Pigs can breed and need no pasture but require milkers & cheesemakers, etc.
So the only way to provide food in an eternal fort is with a vampire doing the food production. Ditto booze (though they could drink water instead) and clothes (as new dwarves would have to be assigned to a squad to get metal armor). And for any other luxury items that a fort might need.
Vampires require blood which fortunately, unlike food, can be produced without any player input for an eternity, given a large enough herd.
So I think the only possible Eternal Fort would consist of one or more vampires producing enough food, booze and clothing for a large (turtled) fort of dwarves, who just carry stuff around, eat, breed and sleep - with one of them every so often falling asleep in a masterwork gem-encrusted and slightly bloodstained bed laid out in the vampire burrow. The only limit to how long it would last would be how many coffins you could pre-build, and of course whether there were enough luxuries around to compensate for the endemic, unending murder.