Within the general settings for 3dcart in Shiptheory, you're able to specify your "Order Processing days" and your "Dispatch cutoff time":
This information is used to pass the estimated delivery date back to 3dcart. Changing this will alter the delivery date for any future orders put through, and of course the cutoff time will also affect the date passed back.
For example, if you processed an order after your cutoff time in Shiptheory, this would mean that the estimated delivery passed back to 3dcart would be one day longer due to the order not being able to be dispatched until the following day.
These fields won't affect any information in Shiptheory, but they will solely effect the delivery date in 3dcart.
If you have any issues or questions regarding this, or would like any help getting set up, please feel free to contact support.