Bug with date filter not taking well formatted dates containing only integer like 2018

I will take this opportunity to thank you all for the great work on liquid.
I was reading the documentation here

https://shopify.github.io/liquid/filters/date/

and assumed that the statement

"date works on strings if they contain well-formatted dates:"

meant that any output of the date filter could be used again as input for the date filter. So I expected that this would work and it does

now | date: "%B %Y"

and then something like

"March 2018" | date "%Y"

works and yield 2018 but this does not work

"2018" | date "%Y"

is this a misunderstanding of what is meant by well-formatted dates or it should work?

Start with the date filter documentation linked in the issue and reproduce the examples for "March 2018" and "2018". The payload names no source file or test; done means the handling of integer-only date strings is resolved and the documentation or behavior clearly agrees with the result.