This would be a really cool thing to be implemented into your game. I would go with Roblox Games because you have to purchase an API to use real life stocks in your game (correct me if I’m wrong about this). @boatbomber actually made the real life Roblox stock in his own game and I think you will be interested in this:
Anyways, to get the time, I would send the current os.time to a datastore when the player purchases the “stock”, and then you can subtract it from the os.time when the player sells it to see how long it took.