Specifying Time Across Celestial Reference Systems: A Multiscale Framework and an Earth–Moon Case Study

Civil timekeeping originated in terrestrial cycles, encouraging a linear intuition in which event order, elapsed duration, and motion are represented on a single, implicitly universal axis. This paper examines the limits of that intuition in deep-space science by distinguishing civil time, atomic time, proper time, relativistic coordinate time, and cosmological time. The central problem is not the use of seconds, terrestrial years, or light-years, but the omission of the reference system, observer worldline, gravitational model, signal-propagation model, conversion convention, and uncertainty. Existing geocentric, barycentric, and lunar coordinate-time standards already implement a layered approach. A quantitative Earth–Moon case study shows that an uncorrected mean clock-rate offset of 56.02 microseconds per day corresponds to a one-day light-ranging bias of approximately 16.79 km. An eight-component multiscale reference-time specification and an empirical evaluation protocol are proposed for deep-space event records. Current theory and experiment support the vacuum speed of light as a local invariant of causal structure; alternatives require mathematically defined and falsifiable predictions. The proposed framework does not replace terrestrial units, but makes multiscale cosmic-time statements computable, interoperable, and empirically testable.