The total time spent by visitors on the website.
Duration is calculated as the sum of time differences between individual visitor actions.
The calculation is based on timestamps which are set only at the start of each action. The missing time stamps at the end of an action mean that the duration of the last action in a visit cannot be calculated. Similarly, it is impossible to calculate the duration of a visit with only one action.
Thus, the measured duration is always shorter than the real duration.