Suggestion 

Provide an option to extract the date when a user was granted product access.

• Allow the date to be included in the user management (CSV) export
• Allow the date to be extracted via user management API.

Workaround

1. Use the product audit logs to find when a user was added to a group that gives product access - Jira admin or Confluence admin 
2. Use the Organization audit log (Require ACCESS subscription) to find when the user was added to a particular product.