Finding the time (in seconds) since the user last opened the app

I’ve been looking at the System.DateTime class to figure this out. But that seems highly exploitable. I’m trying to figure out a system where the game checks the time since the player last opened the app, but I would like to prevent the user from just setting the time on their computer to a few hours in the future and bypassing this. Does anyone know of a work around?

If you don’t trust the user’s time, you should probably ask the time from an on-line time server, or a dedicated server of your own.