How long has a Computer been in a smart group

No way from Jamf side to get this information.

Create two Extension Attributes instead.
Every recon they check the Classic API wether the client was added to the Smart Group or removed from it. Store this information in a plist and read it from the EA.

@Cayde-6 Upvote the Feature Request Add the ability to check for when a Computer was added to a Smart Group if you want to let Jamf know this capability would be useful.