I admit I haven't explored anything with dates/time in Pico8, so just wondering out loud if there's a way to save a timestamp so that you could use it later and have your game behave like it "did something" during the off time.

For example, hatching an egg. You start the game on Monday and quit. Saves the timestamp. You play the cart again on Thursday. The timestamps are compared to determine how many days/hours/whatever have gone by and routines are run against the egg accordingly...on its way to hatch or something.

I can't think of a native Pico-8 API that would allow you to do this. If your target is the web player, you could augment the player with a bit of JavaScript that tells the player the current datetime through the GPIO pins.

(If your target is PocketCHIP, you could use the actual GPIO pins and an external clock circuit. ;) )