As far as I know the GPL does not allow trial periods and asking for money later on, but feel free to correct me. That's why elementary cannot do such a thing.
Yet some projects still squeak out some money: Synergy, DOSBox Turbo, etc.
Freedom 0: "The freedom to run the program as you wish, for any purpose."
Does this line make a program un-GPL?
exit(0) if Time.now > Time.new(2016,11,15,0,0,0)_that's why it won't work_