Why scientific articles do not include year/date? I noticed that (most?) articles do not include the date of publishing/writing.

For example see this paper (on researchgate.net).

If I click for a citation on that page, I see there the year listed as "2018". But in the article itself, there is no indication when it was written.

Why is that? Or did I miss something?

PS: To clarify, I am not asking about the duration of the work, but the information when the work was done (realizing that a single point in time can not be picked). For example if there is a paper about "Method for better battery capacity", then it is a substantial information whether it was written or published in 1899, 1989 or 2018.
For most purposes, the publication date is more important than the date the work was done. There are some exceptions, but they are few. The author doesn't know the publication date when the work is done. 

The publication date is a kind of meta-data about the publication, not precisely relevant to the work itself (most cases). That meta-data is maintained by the publisher, of course. 

Some work, of course, is time dependent and so the authors will, then, likely say  when data was gathered, and so on.