Actionscript 3 :: Unlimited Map Dimensions For A Game?

Recently I've been planning out how I would run a game with an environment/map that is capable of unlimited dimensions (unlimited being a loose terms as there's obviously limitations on how much data can be stored in memory, etc). I've achieved this using a "grid" that contains level data stored as a String that can be converted to a 2D Array that would represent objects and their properties.Here's an example of two objects stored as a String:

"game.doodads.Tree#200#10#terrain$game.mobiles.Player#400#400#mobiles"

The "grid" is a 3D Array, of which the contents would represent the x/y coordinate of the grid cell. The grid cells would be, say, 600x600.An example of this "grid" Array would be as follows:

var grid:Array = [[["leveldata: 0,0"],["leveldata 0,1"]],
[["leveldata: 1,0"],["leveldata 1,1"]]];

The environment will handle loading a grid square and it's 8 surrounding squares based on a given point. ie the position of the Player. It would have a function along the lines of

function loadCells(xp:int, yp:int):void

This would also handle the unloading of the previously loaded cells that are no longer close enough to be required. In the unload process, the data at grid[x][y] would be overwritten with the new data, which is created by looping through the objects in that cell and appending each new set of data to the grid cell data.works fine in terms of when you move in a direction, cells are unloaded/saved and new cells are loaded. The problem is this:

Say this is a large city infested by zombies. If you walk three grid squares in any direction and return, everything is as you left it. I'm struggling to find a way to at least simulate all objects still moving around and doing their thing. It looks silly when you for example throw a grenade, walk away, return and the grenade still hasn't detonated.

I've considered storing a timestamp on each object when I unload the level, and when it's initialized there's a loop that runs it's "step" function amount of times. Problem here is obviously that when you come back 5 minutes later, 20 zombies are going to try and step 248932489 times and the game will crash.