The NAV that comes new with systems is usually a limited version and you need to extend the subscription... by paying for it. Even the NAV that you purchase through the store is usually a limited time version for the program and data definitions (check and make sure that it is supposed to be a year version -- but I do not think that it is). 3 months is about right. Through the store, you are paying for the program.
You must subscribe for additional months, and one way of doing that is through the web.
When I renewed through the web, they calculated based on their date/time info (there was some hours difference) on my receipt, but essentially the date and everything else was correctly added. They will add a year if you choose to subscribe for that period.
If you believe the subscription info on your computer is incorrect (AFTER reading the documentation that came with the software), you should be able to go through their subscription troubleshooter to resolve this:
http://www.symantec.com/techsupp/nav/nav_2003_tasks.html
Need help with "solving a software issue"
And i want "to solve a subscription issue"