I might be missing what you're looking for but isn't there a a Last Check-In and Last Inventory field in the general tab of a device's inventory by default?

That is it,  I do see that it shows different when running a search.  It just shows up as "Last Update".  I am still learning Jamf - Thanks for your quick response.

Jamf Pro reports this automatically and is updated in the general record "Last Inventory Update".  Reporting this via Extension attribute would need a recon to take place anyway...so it's really not needed.