As Cessna are an American company they are bound by American regulations.
(1) Section 91.417(a)(2)(i). Requires a record of total time-in-service to be kept for the
airframe, each engine, and each propeller. Part 1, section 1.1, Definitions, defines time in service, with respect to maintenance time records, as that time from the moment an aircraft leaves the surface of the earth until it touches down at the next point of landing.
http://www.faa.gov/documentLibrary/m...ar/AC43-9C.pdf