I don't know how serious e621 is about tagging images with years, but I'm just going to throw out my two-cents on the matter.
I see a lot of users tagging images with the current year:
-Without knowing a source or it's origin date.
-When the source says otherwise.
-When an artist is uploading past works. (Mistaking reuploads for new works.)
Off the top of my head:
https://e621.net/post_tag_history/index?post_id=675379 (Sourceless image tagged 2015, actually from 2013. If you don't know, don't tag it.)
https://e621.net/post_tag_history/index?post_id=604725 (Tagged 2015 when source says 2012. Uploader gave the source upon posting, so no excuse there.)
I personally think an image should not be tagged with a year unless it's signed on the image itself, or if it can be confirmed as a new/recent work.
It's nice to have years as tags so someone can see an artist's progression, but there it not much point in tagging something with a year if it's wrong.
Updated by user 59725