I think anything can be cracked with enough time.
One thing you could do is make the hacker's life more difficult by creating a complicated logic inside your app, literally a cypher where only you knows where the key elements are found so even once decompiled, the code will not have an obvious neon sign saying where your time variable is.
Then you can top this with options to scramble your data that's saved in the file, with a hash. Be careful though because there are legal regulations regarding the use of encrypted data but if it stays local and minimal, if data is not exchanged and not sent outside of one's device, it should be fine.