Elysia cron Execution time vs Shutdown time
The documentation surrounding that variable is pretty much non-existent. This is how it's calculated, in elysia_cron_completed():

time() - $cron_completed_time


Where $cron_completed_time is a global, presumably a timestamp indicating exactly when an associated cron job ended.

I'm afraid you'll need to infer exactly what that timestamp is from the above; I think it refers to the gap between when a cron job was completed, and the time it was logged in the database.