It's not an accurate indication of how much time you've spent using APS. All it can measure is the time spans between when you load pages on the site. It assumes that if you load two pages within a span of 15 minutes, that the entire time in between was spent using the site. If you only load a page once in any 15 minute time span, the site assumes that you spent 15 minutes online for each page you loaded.
The net result is that the clock is a serious over estimation of your time spent here.