To create start and end times, you're going to have to first create a time block. This will allow you to start and end groups of items at different times. If you edit a time block, it will update the time for every items listed in that that block.
1) Click "Auctions" on the far left.
2) Click on "Manage Auctions".
3) Click on the Name of your auction.
4) Click "EDIT".
5) Click "Yes" by "Set auction time" to add or edit time blocks.