How are the hours that count toward fatigue limits calculated by the FAA?
Flight prep and other non-flying duties are accounted for, but not under “flight time.” There are seperate regulations for time “on-duty” and time actually flying.

Flight Duty Period

The flight duty period begins when a flightcrew member is required to report for duty with the intention of conducting a flight and ends when the aircraft is parked after the last flight. It includes the period of time before a flight or between flights that a pilot is working without an intervening rest period. Flight duty includes deadhead transportation, training in an aircraft or flight simulator, and airport standby or reserve duty if these tasks occur before a flight or between flights without an intervening required rest period.

Flight Time

The regulations define flight time as: when the plane is moving under its own power before, during or after flight.

Rest period

Rest periods are “measured from the time the flightcrew member is released from duty.”

Cumulative hours

Cumulative hours are limited for both flight duty period and flight time. There are 7 day and 28 day limits on FDP; there are 28 day and 365 day limits on block time. The 7-day and 28-day limits are expressed as hours, so a rolling 168 hour limit (7 days) and rolling 672 hour limit (28 days). [thanks Ralph J]



This info was taken from the FAA Fact Sheet – Pilot Fatigue Rule Comparison