The Grandfather Clock is a furniture item that when right clicked, displays the current time, to the minute like a Gold Watch or a Platinum Watch. The inner pendulum is gold, and can be seen behind a pane of glass when placed. They can be found in Underground Houses as of 1.1. They are 5 blocks (10ft) high.
The Oxford English Dictionary states that the 1876 song My Grandfather's Clock is responsible for the common name "grandfather clock" being applied to the longcase clock.