What's the best way to calculate time passed since last playing app?

Questions & Answers
1

Basically to do a Farmville crop growing over time type system.

I would guess save the system time in a PlayerPref on minimize and then check it when you run next?

Any other suggestions or can anyone point me at the functions to do that?

2

PlayerPrefs are only stored on the players computer. Is would be better to store information at a server.

Take a look at the wiki: Server Side Highscores

Good starting point with SQL, PHP and JS code.