I looked over suggestions and pending features I didn't notice it being in the works.
Right now if you use a bulk tool to select watched episodes it says "Watched on: Unknown". And if you select each episode individually it uses the current date.
My suggestion would be to add a manual way to enter the date for episodes that were added via the bulk tool. It's a low priority suggestion, especially since the stats are only available up to a year right now. But with more user statistics being a pending feature it would be very welcome.