UPDATE 9/12/2007 – Google has reversed its counting methodology for time on site.
We attended the GA Conversion Universiy last week and it was discussed why Google Analytics time on site numbers have jumped.
Google’s explanation is that Google Analytics is no longer counting visitors who bounce in their TOS calculation. They weren’t sure when the change took effect (they thought end of July). The Google folks agreed it should have been announced.
I think most agreed that this was a better way of measuring time on site becaues the way it was explained is that TOS is calculated is looking at the time between one page is called to the next. With those that hit the homepage and leave, they were getting zero values, bringin the TOS down – even if they spent a couple of minutes looking at your homepage, then left.