Hi Chris, Malcolm,
We do take the updated date from the editor tracking attribute if editor tracking is turned on. For hosted services, we take the last edited date on the hosted features. For services that are not hosted and do not have editor tracking, we default to the ArcGIS Online item's last updated date.
For services that have formal metadata attached and has a published date, we show both the last updated date and the published date.
Let us know if you have any other questions!
Courtney