Also the supposition that the age of an item tells you when it was lost is spurious at best, if Marty drops the broach in the car park of the Alberta College of Art and Design, nobody finding it would assume that someone from the 14th century had recently parked their car there! Things, especially jewellery is often passed from generation to generation, a 14th century heirloom could easily be dropped by someone from any period. Still, they have to keep the story going I suppose.