If you need to know when a specific inventory item was added to WholeCell, you can find that date on the item's page. Click on the item's ESN or ID to get to its page.
On the upper left side of the page, you will see what PO the device is in, the company that sold it to you, and when it was added in WholeCell.
To find the inventory added date on many items you'll need to create an Inventory Report. Here's how to create one: http://help.wholecell.io/en/articles/2813756-how-inventory-reports-work
Create an inventory report that includes the 'Inventory Date' column. On the Edit page of the report, the bottom section shows the columns that you can add to it, make sure to check the 'Inventory Date' check box. Filter for the inventory you want to see the added dates of, save the report, and you'll see the added date as a column.