date stone1
/ˈdeɪt stəʊn/noun
The stone of a date.
Origin
Late Middle English; earliest use found in Liber de Diversis Medicinis. From date + stone.
date stone2
/ˈdeɪt stəʊn/noun
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.
Origin
Early 19th century. From date + stone.