What is "normal business hours" for 31-205-46

What is the interpretation or normal business hours for this cost principle application?

FAR 31.205-46( "Airfare costs in excess of the lowest priced airfare available to the contractor during normal business hours are unallowable except when such accommodations require circuitous routing, require travel during unreasonable hours, excessively prolong travel, result in increased cost that would offset transportation savings, are not reasonably adequate for the physical or medical needs of the traveler, or are not reasonably available to meet mission requirements. However, in order for airfare costs in excess of the above standard airfare to be allowable, the applicable condition(s) set forth above must be documented and justified."