What date should I put on a paper that I hand in to my teacher?

I know that it is important to have dates on papers that you hand in to teachers. I usually put the date that I start the paper, but sometimes I change it to make it seem like I'm not procrastinating. I've also used the date the paper is due. No teacher has ever said anything to me or taken off points for how I have the date.

So my question What is the correct date to put on a paper?