UTC is defined on the rotating “geoid” which is the modern version of mean sea level. If you happen to be NIST (one of the USA’s principle time labs) or Schriev

UTC is defined on the rotating “geoid” which is the modern version of mean sea level. If you happen to be NIST (one of the USA’s principle time labs) or Schriever space force base (which hosts the ground-based reference atomic clocks for the GPS satellite fleet) you need to adjust the speed of your clocks slightly to compensate for being in Colorado 1 mile above sea level.