Note 1 to entry: See also "time".

Note 2 to entry: In order to compare the value of a proper date given in frame S where the clock is in rest, with the date in another frame S′, frames S, S′ must be synchronized.

Note 3 to entry: This definition is valid also in non-inertial frames.

Note 4 to entry: The coherent SI unit of proper time is second, s.