This is about modifying a one-time schedule block. For help creating one, see How Do I Create a One Time Event on my Schedule.
You are now able to easily modify a one-time schedule block from the screen invoice. The date stamp at the top will indicate if the event is booked into a one-time slot or not.
In this case, the "Change Date/Time" button will respond differently. Instead of taking you back to the schedule to pick a different time slot, it will bring up a form to manually specify the date/time for the event.