There have been many requests for this (or something similar to this) and this will be needed with the eventual goal of easy expiring of watchlist items.
It should be decided if this should be a timestamp of when the item was added to the watchlist or if this is the timestamp the watchlist item was last watched / touched? This will be important when tags are possible? When I add a tag should the timestamp stay the same or change? Page moves should also be considered.
This has been extracted from my comment at T124752#1998193