Along the lines of T7347 T21992, MediaWiki should wrap the output of ~~~~ and ~~~~~ in <time> elements.

I wrote a user script (currently installed as a gadget at the Vietnamese wikis) that hunts for output from the ~~~~ and ~~~~~ syntaxes and turns them into dynamically-refreshing, relative timestamps in the user’s local time. The most expensive part of this script is likely the NodeIterator, which would’ve been unnecessary had the timestamps been marked up somehow.

Marking up the timestamps might be complicated by the fact that they’re sometimes used in template parameters, parser function parameters, Lua, etc. So I’m not sure at what stage the markup should be introduced, or whether a new syntax should be introduced for talk page signatures (yikes!).