I came across a question today which I had previously upvoted, however I was unsure when exactly this was.
Currently when you hover over a upvote arrow the tooltip displays:
This question shows research effort; it is useful and clear (click again to undo)
This is the same regardless of whether you have upvoted the post or not.
If you have upvoted/downvoted, would it be possible to display the date/time at which this was actioned?
I would find this useful as most times I come across a post I've voted on before is because I was having a similar problem in the past. Seeing exactly when this post first helped me would help me work out what problem it was before, and could help jog my memory.