When creating a new mobile key through the ADD GUEST button in OpenKey HOST, the default check-out will be set to tomorrow at the standard check-out time.  If you need to reduce the time a key is available to less than that, you can do so by following these steps:

Log into OpenKey HOST and click ADD GUEST on the left panel of the screen.

Select the room. Then click Same Day Check-out.

The Check-out date will automatically adjust to today's date. Click the calendar icon. Below the calendar dates, click the clock icon. Use the arrow down key under the hour and/or minutes to move the time back to the current day at your desired time (2-hour minimum).  

This will change the check-out to the same day, at your desired time.