Hmm.. I don't know if you can see that specifically anywhere..
You could always go to http://blockexplorer.com/q/nethash/
and find a specific difficulty, then copy the block number and search for it on http://blockexplorer.com/
to get the date that the block was solved..
If you know anything about programming you could compute a table for your own viewing pleasure
EDIT: You can set the interval of the blocks on http://blockexplorer.com/q/nethash/
by adding a number after the last '/' (example: http://blockexplorer.com/q/nethash/10