Requested feature overview description.
New "sorting metatags" that arrange search results in order of their year tags.

The tags would be "order:year" and "order:year_asc", along the lines of other sorting metatags like "order:score"

• Putting "order:year" in a search would arrange the results such that posts with the "2020" tag will appear first, then "2019", "2018", and so on. If two or more posts have the same year tag, the most recently uploaded post will appear first. Posts without year tags will appear last.
• Putting "order:year_asc" in a search would arrange the results such that posts with the "1400" tag will appear first, then "1401", "1402", and so on. If two or more posts have the same year tag, the most recently uploaded post will appear last. Posts without year tags will appear last.

Why would it be useful?
Presently, you can't conveniently look at an artist's gallery in order of when each piece was created.
The closest thing we have to this ability right now is "order:id", but that arranges posts in terms of upload date on this site, not necessarily their creation date.
If you want to find an artist's oldest or newest piece, you have to visit every post, visit the post's source, check the date there, and search for an older/newer piece either at that source or on e621.
This process could be made significantly easier if users could tell e621 to automatically arrange posts in order of their creation date.

However, as e621 reasonably doesn't have metadata for the exact day a post was made, the meta tag for the year a post was made will have to suffice.

What part(s) of the site page(s) are affected?
The order of search results.