The Longest Off Corp report page has the column header for the first column labelled as "Last Off Corp Preflight".
I am finding this a little contradictory, since the empty label for some clients says "Zero on corp connections", yet the two clients I have implemented the on_off corp check for are reporting the "Last Off Corp Preflight" as their most recent preflight, even though they have never checked in off corp. Further, on the Summary page, the "Connections" for on/off corp % bar shows 100% on corp (which is what I would expect as I haven't tried connecting off the corporate network)