General Relativistic time differences come primarily from differences in gravitational potential, not field-strength. Thus, it is impossible to perform the sug

General Relativistic time differences come primarily from differences in gravitational potential, not field-strength. Thus, it is impossible to perform the suggested procedure. We don't (and probably can't) know what zero potential looks like. Furthermore, we would probably measure those potential differences using clocks that rely upon the SI as presently defined.