Comment by wh313

There's Barycentric Dynamical Time, which takes time dilation into account: https://en.m.wikipedia.org/wiki/Barycentric_Dynamical_Time

NASA has actually thought through this problem and created SPICE, which lets you convert between different time systems, some relativistic and some not, as well as doing conversions between various reference frames: https://naif.jpl.nasa.gov/naif/index.html