Why are dates (of writing or publication) missing in scientific papers?
Scientific papers are usually published in journals and conference proceedings. Publishers report at least the publication year and month, but may also report more information, like the submission date, the revision date and the date of the first online publication.

Your example is just a preprint, not the publisher's version, and does not contain any information. However, you can find the published paper at this link, where you can freely download the pdf with the publication year. You can also read the front matter of the proceedings to find further information.