On the Order Page Spiro will not allow you to place an order for a date and time the system does not think is possible. You can however override this date and time in the admin panel.
Steps to Change Date and Time
Place the Order as the client
Before you submit the order checkmark the button "Do Not Send New Order Email"
Once the order is submitted edit the job
Click the Reschedule Button
Use Option 1 (to keep the status of the job)
Below you can change the date and time of the appointment
Click Save
To send the Email to the client click the "Send Confirmation Email" button