Yes, definitely! To customize the (start and) end dates of an item, click into the item, head to Advanced, then scroll down to adjust the start and end dates.

Note: The actual payment processing and fulfillment of the item will still happen along with the rest of the auction (at the end, when you click "Close Auction", and "Charge Cards"). Having an item end early really just means that only the bidding stops at the time you set.