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?
Regardless of whether it is homework or any other sort of technical document, the correct date to put on a technical document is the date of its most recent revision.  If the history is important, other dates can be added as well (e.g. the submitted/revised/accepted dates on some journal papers). 

As for your professors never saying anything... they likely had no reason or even ability to discriminate between the different systems you used to date, and it likely did not matter for the purposes of the class.