Sorry I didn't reply to the previous post. This is actually on my list to implement for 1.8, when I get to it.
As for your question, right now one way to estimate the vote dates would be to use the dates of the actual posts voted on, since people vote mostly on newer posts. (In fact that may be what I do when adding this to Q2A.)
Edit: forgot to add, you can also make an event plugin that stores the date of the vote in a separate table. A little bit overkill since you would basically end up duplicating the qa_votes table, but that's the only way to do it accurately for now.