For requests where the travel mode is driving: You can specify the departure_time to receive a route and trip duration (response field: duration_in_traffic) that take traffic conditions into account. This option is only available if the request contains a valid API key, or a valid Google Maps APIs Premium Plan client ID and signature. The departure_time must be set to the current time or some time in the future. It cannot be in the past.
From Google Matrix's Documenation:
For requests where the travel mode is driving: You can specify the departure_time to receive a route and trip duration (response field:
duration_in_traffic) that take traffic conditions into account. This option is only available if the request contains a valid API key, or a valid Google Maps APIs Premium Plan client ID and signature. The departure_time must be set to the current time or some time in the future. It cannot be in the past.