I'm creating a game on Iphone/Android. In my game, user plants a tree then have to wait for that tree grow up. Each time user opens game, I check the time when user planted the tree and the current time, then decide the tree is grew up or not.
The problem is, if user plants the tree, then change device's time, he/she can cheat to make the tree grow faster.
How to avoid this? Is there a way to get the "real world current time" which user can not change?
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