Sometimes I come across an old question or answer that I voted on in the past, but I don't clearly remember when (or sometimes even why) I voted it up or down. Being able to see the date/time that I cast a vote would help me remember my mindset at the time.
When mousing over the toggled vote, the tooltip reads the same as if I hadn't voted, with no further information. For example:
This answer is useful (click again to undo)
The only way to get the timestamp is if I try to click the vote to undo it and get the locked vote message:
You last voted on this answer [datetime] Your vote is now locked in unless this answer is edited
However this requires me to guess that the vote is locked - if there was an edit since then I'll actually undo the vote and erase the info I was looking for.
Feature request: Please add the vote timestamp to the tooltip when mousing over a cast vote. This is already done with the green acceptance checkmark for example.