I tried copy and pasting the string The concert starts at 20:00. from https://developer.mozilla.org/en-US/docs/Web/HTML/Element/time
in this diff: https://en.wikipedia.org/w/index.php?title=User:Quiddity_%28WMF%29/sandbox&diff=668495457&oldid=668240695
and it was saved with the full rich text as The concert starts at <time>20:00</time>.
Related discussion at T34545: MediaWiki should support <time> in WikiText (the request filed in 2011 for Wikitext to support the tag).
There are (as of today) 14 instances in mainspace at Enwiki: https://en.wikipedia.org/w/index.php?title=Special%3ASearch&profile=advanced&search=insource%3A%2F\%3Ctime\%3E%2F&fulltext=Search&ns0=1&profile=advanced
At least 1 was added whilst using the tool ProveIt: https://en.wikipedia.org/w/index.php?title=Sonia_Orin_Lyris&diff=prev&oldid=603019095
Others were added with VE, e.g. https://en.wikipedia.org/w/index.php?title=Elizabeth_Scarr&diff=632644339&oldid=632640602
We should decide how to deal with this. (Encourage/define, remove, other ?)
Originally reported at :en:WP:VEF with a pointer to this diff: https://fr.wikipedia.org/w/index.php?title=Bataille_du_d%C3%A9fil%C3%A9_de_Ver%C3%ADn&diff=next&oldid=116144125