Oh I find it so aggravating when I don't have a date. It seems like most academic papers I found on interesting security and programming stuff didn't have dates. I had to pull them off Citeseerx or the school's site. You'd think it would be a requirement in the style guides given importance of when something was published for relevance or context. I've never heard an explanation for why this isn't so.

I do appreciate the authors that put one on there, though. Saves much Googling. :)

Conference papers include a copyright notice with the name (and year) of the conference.

Papers that aren't published at conferences (preprints, technical reports... etc) don't have any standard format for this. Sometimes (especially for preprints) it's left out in expectation of adding the copyright notice in a future version, sometimes it's left out because the document is in a very preliminary stage and hasn't been officially published and sometimes it's left out because people just didn't think of it and there's no standard format that includes a date for things that aren't published to a specific venue.

You don't need a standard format. You just need to put "DRAFT - [today's date]" somewhere in the document.

Im not saying it has to be done a certain way. Im saying my college required the paper to have margins, a title, etc. Im surprised most of them dont require a date of any kind on the document sonewhere. They could write the year in Size 8 in upper-right corner in light grey and that would be better than many in my collection. ;)

The first four digits of arXiv papers links are the year and month of submission (to the arXiv). Just from this link I can tell that this was submitted in September 2016.