Ask HN: what's the most accurate way to measure time on site?

This is probably Google Analytics biggest weakness, as their method of measuring timestamps between pageloads does not correctly account for single page visits, or visitors who come to your website and open multiple tabs of your pages at one go. This is a huge problem, as time on site is one of the primary determinants of where advertisers spend their money.

Could HNers suggest more accurate ways of measuring time on site, and if there is an analytics service using the suggested method, please link to it?