First of all you should decide how the clock will look like.
Here is one example of a good looking clock
http://davidseye.com/DigitalClock/
You can draw a image for the clock and decide how it will animate. Try searching for some good looking digital clocks in the web no matter in what technology( Flash, Java or Silverlight they are made of) then come to your own design.
Please Mark as Answer if this helps you.
Thanks n Regards
~Tanmoy
Blog: http://anothersilverlight.blogspot.com/