I'm developing a mini game that has an Energy System. The maximum energy a player can have at one time is 5. Whenever you execute an action, 1 energy gets deducted. The energy will be replenished by 1 after every 10 minutes.
I can see the easiest way out of this is by querying an external server which will return me a proper time. The problem is that this game is designed for offline play, and it's not meant to query a server every time I launch.
I'm not sure how to go about from here. How should I prevent a fast forward time cheat so that players will not get energy easily?