Without trying to dodge the question, there are some good javascript tutorials available on the web. Google "javascript tutorial."
If you google "javascript countdown" or similar, you will also find some decent examples.
These days, though, you might want to replace "javascript" with "jquery." Jquery is a javascript library -- and a quite extensive one -- that simplifies the use of javascript. Support of jquery is built into DotNetNuke. There is a nice example
here.