The Open Ports table, shown when you click on an asset in Inventory>Assets, shows all the open ports on the asset, together with its First Seen and Last Seen dates.
The Open Ports table is a snapshot, not a historical log. We roll up observations so that each unique IP:port appears once with its service, along with First Seen and Last Seen timestamps. First Seen records when that IP:port was initially observed, and Last Seen is the most recent time it was reported.