We've had this since the 90s: https://en.wikipedia.org/wiki/Barycentric_Coordinate_Time https://en.wikipedia.org/wiki/Barycentric_Coordinate_Time But it's not

We've had this since the 90s: https://en.wikipedia.org/wiki/Barycentric_Coordinate_Time https://en.wikipedia.org/wiki/Barycentric_Coordinate_Time But it's not possible to directly measure your own coordinate time relative to a distant reference frame, so there has to be some way to express proper time at your location. And for roughly-comoving observers at a given location (say, on the surface of the same astronomical body) to coordinate their measurements of proper time, they need a common standard.