During the past couple of centuries, it hasn't been uncommon to design a clock or watch in a way that reminds you of your own morality, "memento mori". This silver skull watch from the late 18th century is a particularly well-made — and morbid — example.
With a hinge that lets you open the skull up to see it's ticking watch-face brain, this pocket watch was probably made sometime in the 1780s. While the theme "do not forget that you are a mere mortal, and are going to die, like, no matter what" is not historically uncommon, actual pieces like this are, and can get pretty expensive nowadays.
Art of Mourning quotes the Taft Museum as describing the concept this way:
The skull and watch are part of the standard subject matter of 17th-century vanitas still lifes. Vanitas is from the Latin for "emptiness" or "untruth," from which comes the English word "vanity." Such pictures depict objects that have an underlying moral message-usually about the fleeting nature of physical reality.