Save yourself the five bucks and check out these articles (it's a do it yourselfer)
http://www.sentra.net/tech/garage/engine.php?#Timing
http://www.nissanperformancemag.com/...8/timing.shtml
What the articles don't tell you is what each mark on the crank pulley means. Looking at the pulley, the marks starting on the firewall side going from left to right are -5, 0, 5, 10, 15, 20. So if you wanted to advance to 15 you would put it on the second mark from the right.
Hope that helps.