Hi Phil,
What that date represents will depend on your service:
* If the service is hosted on your own server and you have editor tracking turned on (Enabling and managing editor tracking—Help | ArcGIS for Desktop ), that date should reflect the latest date on the editor tracking field.
* If the service is hosted on ArcGIS Online, we should date the last edited date from the hosted feature service.
* If the service is hosted on your own server and you do _not_ have editor tracking turned on, the date will reflect when the ArcGIS Online item itself (from an edited description, title, etc).
There is one bug we are currently working on related to hosted feature services, where sometimes they will show the wrong date.
Courtney