The stone of a date.
Late Middle English; earliest use found in Liber de Diversis Medicinis. From date + stone.
date stone/ˈdeɪt stəʊn/
An engraved stone embedded in a building or other structure, commemorating the date of construction and sometimes bearing other information, as the name or initials of the original owner, builder, etc.
Early 19th century. From date + stone.