Article II, Clause 8
Before he enters the Execution of his Office, he shall take the following Oath or Affirmation:—"I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States." [Notice there is no "so help me God" in the Constitution.]
Amendment XX, Section 1
The terms of the President and Vice President shall end at noon on the 20th day of January...and the terms of their successors shall then begin.