For the auction listing type only you have the ability to send the exact end time. To allow this feature to your sellers check "yes" for the following admin tool setting:

LISTING SETUP > GENERAL SETTINGS > Allow Auction End Time

If you check "yes" for the above admin tool setting your clients will see a choice like this in their placement process:

But do note your clients can still choose a flat duration for the auction by changing the dropdown in the front of that field to the other option as shown below:

Once duration is selected the client will be able to choose from the duration options you allow them.

<tip cn>This configuration is overriden if you choose to "charge by duration" within the price plan for the user placing their auction</tip>