5 U.S.C. § 5550b

(a) Notwithstanding any provision of section 5542(b)(2) or 5544(a), each hour spent by an employee in travel status away from the official duty station of the employee, that is not otherwise compensable, shall be treated as an hour of work or employment for purposes of calculating compensatory time off. (b) An employee who has any hours treated as hours of work or employment for purposes of calculating compensatory time under subsection (a), shall not be entitled to payment for any such hours that are unused as compensatory time.