This is about modifying a one-time schedule block. For help creating one, see How Do I Create a One-Time Schedule Slot?.
With Party Center Software you can modify a one-time schedule slot from the screen invoice. First, you'll want to ensure "Enable Modify One-Time Schedule Slot" is enabled under "Preferences" in "Facility Info".
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.