you can use host.internal.dateCreated attribute. The value represents the date and time when a host was added to the SolarWinds database (for virtual machines it's vm.internal.dateCreated attribute).
FYI in VMAN there are two trends showing requested information. Go to Reporting - Business Views module and choose one of trends mentioned below:
Or you can run predefined "Newly Added Hosts" and "Newly Added VMs" search queries.
Did it help?
Thanks for the suggestion--it was exactly what I was looking for. Unfortunately, it didn't actually yield the desired result; for whatever reason, all but one of the servers show up as having been added at a date which is much later than when they actually came online. We had some issues with our VMAN installation, and I think that may be why the date is off. Back to the drawing board . . .