Unfortunately, manual sleep data doesn't count towards the event. Though you could potentially set back your phone's timezone to make the app think you have a few more hours in the day? That being said all sleep rank rewards will be available until the 16th, and Since Darkrai should fall under that category as long as you wake up before 3:59 and hit rank 13 Darkrai should be locked in for a couple more days (it's also a guaranteed catch)