". It is impossible, in principle, to deduce, from WITHIN a work of art, when the work began UNLESS somehow "outside" information gets in. "

Not really. With some scientific analysis you could deduce when a picture was painted. You could use the wooden frame to establish a dendrochronology, the fabric of the canvas will carry debris such as pollen that indicates a certain climate and location, the weave of the fabric will be unique in a certain place and time and the constituent materials of the pigments used will almost certainly be able to be traced and indicate their origin. All someone has to do is apply their mind to the problem.