(1) In the game, click on "Archive", and click on the very last move (even if it be opponent's move). We can get approx. date from the timings of the comments there (n months ago)
(2) get "Start" date and "Time" (1 move per n days) under "Details".
Add n x 2 days to start-date.
Where can you find what date a vote game ended on? It's hard to calculate group ratings without it.