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. Right click on Grandfather Clock makes it tell the time.
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.