Actually, the private message hover titles are already formatted using reusable code (that uses the user's own date format). The public chatter messages are formatted differently in a way that doesn't make sense for private messages (public chatter is never more than 500 seconds old so including the hour, much less the date, is a bit redundant and would require timezone awareness) and so doesn't need to be used anywhere else (at least not yet).

PodMaster created a patch (quite a while ago) to show timestamps on private messages but didn't follow the formatting done in the chatterbox (it includes the timestamp as "tye said {message} on {date}"). In the pmdev wiki I mentioned that I didn't like it because it has the dates unaligned (and adds not a small amount of clutter compared to the old/current presentation) but that I can understand some wanting all of the dates visible at once (instead of in hover titles like I was hoping for). theorbtwo also contributed to the discussion.

So far, this thread makes me switch back to more strongly favoring the hover titles just so I don't have to worry about changing the look of a node (and the inevitable complaints that result).

But I haven't seen even close to a consensus on the design (I'd only seen 3 takes on the matter, including my own) nor a patch that I feel is appropriate to apply. Recent events have reinforced my reluctance to apply patches hastily, especially when they affect how the site looks.