The issue of suspension of the executive power
In 1916, the State Department determined that "there is no interval between the term of one President and the beginning of his successor, although there may be a slight interval when the executive power is suspended." Therefore, a delay in taking the oath of office would not leave a hiatus in the office of the President, but the new president would not have the constitutional power to perform any executive function until the oath of office was taken.[2]
Such finding was based on a 1821 ruling by Chief Justice John Marshall opining that it was "inevitable" the existence of a short "interval in which the executive power is suspended" because "the Constitution only provides that the President shall take the oath it prescribes 'before he enters on the execution of the office'." Marshall then referred to the interval between the midnight of the 3rd of March, when the presidential term started, and the noon of the 4th, when the oath of office was taken, as it was the practice at that time, saying that "there has been uniformly and voluntarily an interval of twelve hours in which the executive power could not be exercised." Marshall further notes that the law was silent on the exact time the oath should be taken, leaving it "at the discretion of the high officer", who could decide to take the oath on the first hour of his term in an emergency, or could defer the taking of the oath until the next day, if more convenient (for instance if inauguration day fell on a Sunday); neither timing would be deemed improper, though it is reasonable to take the oath "as soon as it could be conveniently taken" so to shorten that time interval.[2]
With the enactment of the 20th Amendment, the moment when one term ends and another begins was changed from the midnight of the 3rd and 4th of March to noon on January 20th, but the amendment only dealt with the beginning and end of the presidential term, not with the moment when the new President actually enters in the execution of his office.