The archive is where all your completed tasks are listed.
Use the menu at the top right to go to the archive.
In the "Archive & Stats" overlay make sure you are on the archive page (top left or use keyboard shortcut "A").
Search for completed tasks
You can search for a completed task or project by going to the archive overlay and clicking the search icon at the top right.
Filter and sort
In the search dialog you can filter and sort your completed tasks on the top right.
Find out when item was completed
Underneath each item in the search you can see when the item was completed.