Average time on site and single page views

Hi all, 

I have a two-part question around average time on site and single page views:

Firstly, could anybody explain the calculation for average time spent on site. 

My guess is that it is calculated as follows:

With time spent on site = (time of last image call) - (time of first image call),

Average time spent on site = (sum of time of site for all visits)/(count of all visits) ?

Secondly, are hard bounces included in the calculation of average time on site? Within Google Analytics, I understand that hard bounces are given a value of 0 for time spent on site, does the same apply here or are hard bounces simply not included in the calculation of the average?

For instance, in a scenario where there are 1000 single page views, but 950 are hard bounces, is the average time calculated on the 50 instances that are single page views? 

Many thanks in advance for any help!

Regards, 

Bridgid