I don't think you want a default value. You probably want on load rules. On load:
1. Call now() and populate a field with the time (something like this might work: substring-after(now(),"T"). This rule does not have a condition.
2a. Condition: time > noon. Action: Set the field to today() + 5. You could use addDays.
2b. Condition: time < noon. Action: Set the field to today() + 4.
Hope this helps get you on the right track!