Hey there Ryan,
Interesting question! After some research, I found that the underlying REST service being used for the Drive Time Feature is the Service Area service with synchronous execution.
The Drive Time feature does not pass a time of day or a day for that matter, so the following stands true:
If a time of day is not passed in, the service uses static road speeds based on average historical speeds or posted speed limits. It uses posted speeds in areas where historical traffic information isn't available.
Please let me know if you have any follow up questions!
Keep on keeping on!
