Here are a couple of Trader's Clocks that I've used before. If anybody can suggest any other clocks that are useful for traders then please add them to this topic:

Advanced Timer by Ice Blue

Trader's Countdown Clock

For further consideration: TimeLeft is a countdown, reminder, clock, alarm clock, tray clock, stopwatch, timer, sticker and time synchronization utility. TimeLeft uses Winamp skins or Font to show digits and text. TimeLeft is attractive, easy to use, and highly configurable - it's a really beautiful piece of software.

There are two editions of TimeLeft: free TimeLeft Standard and shareware TimeLeft Deluxe.

Here is a link for the program that bladerunner mentioned:

TimeLeft