Facebook's Ads Insights API returns date_start and date_stop but since they're date objects I can't tell what time zone was used to demarcate the dates. Is it using Facebook's API timezone which they've mentioned to be PT (UTC -8) or is it according to the AdAccount's time zone as I've found mentions of the account's time zone in the Ads Insights API documentation.
The Facebook API does not have the ability to specify a time zone. The time zone is defined at the advertising account level in the settings. Api adopts and returns a date range based on these settings.