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5 U.S. Code § 3349a - Presidential inaugural transitions

In this section, the term “transitional inauguration day” means the date on which any person swears or affirms the oath of office as President, if such person is not the President on the date preceding the date of swearing or affirming such oath of office.
(b) With respect to any vacancy that exists during the 60-day period beginning on a transitional inauguration day, the 210-day period under section 3346 or 3348 shall be deemed to begin on the later of the date occurring—
90 days after such transitional inauguration day; or
90 days after the date on which the vacancy occurs.
Statutory Notes and Related Subsidiaries
Effective Date

Section effective 30 days after Oct. 21, 1998, and applicable to any office that becomes vacant after such effective date, with certain exceptions, see section 151(d) of Pub. L. 105–277, set out as a note under section 3345 of this title.