14 C.F.R. § 135.267
(a) No certificate holder may assign any flight crewmember, and no flight crewmember may accept an assignment, for flight time as a member of a one- or two-pilot crew if that crewmember's total flight time in all commercial flying will exceed—
(b) Except as provided in paragraph (c) of this section, during any 24 consecutive hours the total flight time of the assigned flight when added to any other commercial flying by that flight crewmember may not exceed—
(c) A flight crewmember's flight time may exceed the flight time limits of paragraph (b) of this section if the assigned flight time occurs during a regularly assigned duty period of no more than 14 hours and—
(2) If flight time is assigned during this period, that total flight time when added to any other commercial flying by the flight crewmember may not exceed—
(e) When a flight crewmember has exceeded the daily flight time limitations in this section, because of circumstances beyond the control of the certificate holder or flight crewmember (such as adverse weather conditions), that flight crewmember must have a rest period before being assigned or accepting an assignment for flight time of at least—
[Docket 23634, 50 FR 29320, July 18, 1985, as amended by Amdt. 135-33, 54 FR 39294, Sept. 25, 1989; Amdt. 135-60, 61 FR 2616, Jan. 26, 1996]