you can use the Date plugin to get the "Now" expression. It's the number of milliseconds since 1970. So it always increments; If you save it when closing the app using local storage, and you get it again when the app opens again, you can subtract the new value from the saved value to know the time that passed since the last time you opened the app.
See my little video on the date plugin en the "Now" expression on my youtube channel.
The channel has many tutorials on how to create games, and i'm planning on adding an idle clicker game or something any time soon that will use the same mechanic.