Much easier than you think....
Look for the link labeled User CP (should be towards the top of the page) and click. Locate Edit Signature and click. Type what you want in your signature. For example, my signature box has
11/2006 NCL Sun
12/2004 NCL Dream
12/2001 NCL Sky
and that is what/how it is displayed after each of my posts. No need to input the line, it is automatic. You can then preview your signature and save when you are happy with it.
If you want a countdown clock, here is a sticky that tells how to insert one. Read the entire thread, some updated directions could be towards the end of the thread. When you get the code created for the countdown clock, you would paste the code into the signature box along with any text you want displayed.