This document explains how to  change the Due Date and Time at Checkout

Koha uses the Circ and Fines Rules you have set in Administration, and the branch calendar,  to calculate the Due Date and Time for each checkout but there is an option in the checkout window for Checkout Settings where you can set a default Due Date and/or Due Time to override the system settings. 

On the Checkout window, click on the option for Checkout settings

You can enter a date but you should click on the date widget

Use the sliders for the Hour and Minute to set the required Due Time and click Done.

You also have the option to tick the box to Remember for session.