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)