Armageddon Clock is a Magic: the Gathering card that, basically, deals more and more damage for each player every turn, though players may try to prevent that.
This was introduced in Antiquities and also appeared later in 4th Edition, I think. It is highly annoying and, I think, considered broken and banned.
The clock applet (ArmageddonClock.class) had a normal red 24-hour number display on black background; but each number in it faded from red to black independently at specified speed - so that at least some numbers could be read at same time. Cool blinkyblonky effect! Accept no substitute! R0x0rz, d00d!
Stuff to come: Double buffering and thus faster fades =)
Also, I sometimes use this term to refer to any clock that shows remaining time to some event (there was one at the kuro5hin.org main page in September 2000 when the site was coming back after downtime). Often accompanied with a sign "The end of the world is near!"