TAI conceptually is time measured by a perfect atomic clock running exactly at the geoid. There are some issues with this concept:
- A perfect atomic clock does not exist.
- Older and presumably less accurate atomic clocks are regularly replaced with newer and presumably more accurate atomic clocks.
- Few, if any, atomic clocks are at sea level.
- Mean sea level is close to but not the same as the geoid.
- The geoid is not perfectly known.
This means that the averaging and scaling used to combine atomic clock outputs to generate TAI needs to be updated as older, less accurate atomic clocks are replaced with newer, more accurate atomic clocks, and as knowledge of the geoid improves.
That said, errors made in the TAI timestamps of past events due to imperfect clocks remain errors forever. TAI is not updated retroactively.