Duty period means the period of elapsed time between reporting for an assignment involving flight time and release from that assignment by the certificate holder. The time is calculated using either Coordinated Universal Time or local time to reflect the total elapsed time.
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Now, for the pilots (aka, flight crewmembers)
Sections 135.263 through 135.273 of this part prescribe flight time limitations, duty period limitations, and rest requirements for operations conducted under this part as follows:
(a) Section 135.263 applies to all operations under this subpart.
§ 135.263 Flight time limitations and rest requirements: All certificate holders.
(b) No certificate holder may assign any flight crewmember to any duty with the certificate holder during any required rest period.
So, what is duty (any duty)?
From FAA's Chief Council's Legal Intepretation to Norman Robinson, 2011
Section 135.263(b) states that "no certificate holder may assign any flight crewmember to any duty with the certificate holder during any required rest period." The FAA has consistently interpreted duty to mean "actual work for a certificate holder, or the present responsibility for work should the occasion arise."