How to structure a game that continue even when closed?

Question

I'm developing a Unity based game purely for fun but I'm stuck with a problem. How can I implement a system that keep my game running even when is closed? I don't want to HAVE IT running when it's closed, but I was thinking to add some kind of timer and compute the logic of the game with the time difference between (LAST_SESSION)-(ACTUAL_SESSION).

But how can I implement this? The thing is, how can I "accelerate" the game when I open it so that it's current state is equal to what would have been if I would not have closed it?

EDIT: The game is a sort of RTS. You control a base that has some number of entities that can move independently. Each entity has some kind of job like mine minerals, defend the base from enemy AI, build a structure. The mechanics are the interactions between each entity and the world.

I have a main loop where I update every entity, one at a time. I was thinking that I could do some kind of Load function that calls that main loop with a time_elapsed variable and with every iteration lower this variable by Time.deltaTime or something like that, and when it reaches zero I can start the game.

Sorry for my bad english.

Answer 1

The proper way to do this is, when you exit the game, save the current time somewhere in your project. My .Net skills are awful, but a quick google search suggests System.DateTime.Now.

When the game opens again, check again the time with System.DateTime.Now and compare it with the saved one to calculate how much time has passed. Based on the result, apply your logic for what every object in the scene has been doing all this time.

Pseudo-code:

void OnExit() {
    now = SecondsSinceEpoch();
    write("file.save", now);
}

void OnOpen() {
    last_time = read("file.save");
    now = SecondsSinceEpoch();

    time_elapsed = now -last_time;

    // time_elapsed now holds how many seconds has passed since last time
}

Note that people can change their system time to mess with this.