Something I always wonder with these games, but I stopped trying over a decade ago is seeing what will happen if I forward the device time by 12 hours. Is the game clock set by a server or by my local time? If my device can make it possible to cheat with a time skip, that is definitely something that you should make impossible.
I understand your concern.
However, it's practically impossible to make it impossible to cheat in a single player game for a determined individual.
The main question players must ask themselves is: Do I cheat and potentially ruin the meaning of progress?
Ultimately, this is up to the player, they are the owners of the game, and I'd like to leave it up to them.
Personally, I don't have a desire to cheat in single player games, because if I go down that road, I won't be playing the game for much longer as the meaning of progress would soon be lost to me, and one of the aspects I like most in games is a sense of progress.