No shared "now"
Earth and Mars do not share a single universal present. Communication between the planets is never instantaneous, and the physical distance between them changes continuously — making light-travel time itself part of interplanetary coordination, not an afterthought.
Any comparison between Earth and Mars time must record the coordinate time used, the observed event, the signal transmission and reception times, the light-time value, and the ephemeris model version.
The conversion chain
Precise Earth–Mars time exchange should not be defined as a simple UTC offset. The recommended transformation chain is:
Every precise conversion record should include: input/output time scales, reference frame and epoch, the leap-second table and TAI−UTC value used, ephemeris and gravity-model version, the coordinate time of the signal transmit/receive event, the relativistic delay and light-time model, and computed uncertainty.
UTC is Earth's civil time and serves as an exchange interface with Earth systems — not the definitional basis for MRT. See /research for the relativistic literature (Turyshev 2026, Klioner 2026, Jin et al. 2026) this chain draws on.
Light-time and simultaneity
Earth–Mars light-time varies continuously with orbital geometry, from roughly 3 to 22 minutes one-way depending on the planets' relative positions. Light-time is not itself a time scale — it is the signal propagation quantity between two events.
When displaying light-time: use JPL Horizons or an equivalent precision ephemeris, show the ephemeris and calculation epoch used, distinguish transmit-time from receive-time reference, and disclose whether solar plasma delay, Shapiro delay, and ground-station/spacecraft position are included. A simple circular-orbit approximation should be labeled a rough estimate with appropriate uncertainty.