You could store the day / time using websorage. Do this when the player closes the app (browser object I think).
Then when the player loads the game again, check the value in webstorage with the current date / time and calculate how much time has passed since they last played it..
So if they didn't play the game for 10 hours, and your game gives $1 every hour.. just add 10*1 to their money (obviously money is stored using webstorage too) when the player loads the game.
That's the simplest method.