As it is mentioned on The Google Maps Distance Matrix API - departure time is used to receive trip duration considering current traffic conditions.
I tried to get time between points in different days for example: Fri, 02 Oct 2015 14:10:45 GMT Friday Calculation and in the response duration is 22 min
When I set date with less traffic Sun, 04 Oct 2015 01:10:45 GMT Sunday Calculation the response duration is again 22min.
I think the time durations should be different if in calculations are considered current traffic!