"(So space is measured by time.)" Using the SI units of metres and seconds applies a coordinate system on spacetime. One can take a coordinate interval using

"(So space is measured by time.)" Using the SI units of metres and seconds applies a coordinate system on spacetime. One can take a coordinate interval using that, but careful observation shows that the coordinate system cannot be applied at all scales or agreed upon by all users. In SI terms, the rate of the cycles of hyperfine structure transition frequency of Cs-133 atoms is strictly local to the atoms themselves, and non-local observers will see that rate only if their local Lorentz frame has the same velocity as the atom's, and they are on the same gravitational potential. As we are not massless inhabitants of a Minkowski vacuum, errors accumulate with coordinate distance from the atom used as a source of frequency, and are observable with laboratory equipment at ranges of metres, and are already wholly non-negligible for practical applications (e.g. GNSS) at ranges comparable to the radius of the Earth and velocities comparable to the speed of sound at STP. Sadly, SI predates modern conceptions of 3+1 spacetime and has not really adapted to what we know of our physical spacetime's properties. I think that's more parochial than the base ten issue, personally. :-)