d'you is how do you sounds when English is spoken quickly. The two words melt into one beat: d'you know the time? Writers spell it this way when they want the line to sound like real talking. It is informal. In a test or an email to a teacher, write do you. Though actually, in very fast talk the same letters can also be how did you comes out, so the rest of the sentence (now or past) is your friend. The use you are learning here is do you.