What does the ebony clock signify in "The Masque of the Red Death"?

The clock, which signifies the passage of time, acts as a foreboding messenger of time. Set in the "western or black chamber"--west is the direction of the sunset and is symbolic of the end of life; black, of course, is the color of the shrouds of death--the tolling of the clock marks the passage of time and the approach of the end of life.

Each time the ebony clock tolls, the guests of Prince Prospero become pale and the oldest ones wipe their foreheads that have dampened with nervousness. Even the musicians are required to pause because the chiming is so loud. Finally, the ebony clock strikes midnight and there enters into the minds of the guests a dreadful meditation. Suddenly, there are those who become aware of a masked figure and the guests begin to murmur and feel terror. It is then that the terrible figure of the Red Death appears and the enraged Propero accosts him only to meet his end "within the shadow of the ebony clock."