[yencabulator]

Wikipedia about TT: "It is a theoretical ideal, and real clocks can only approximate it. [...] TT is indirectly the basis of UTC, via International Atomic Time (TAI)."

So, TAI is what we are able to measure. TT is a theoretical construct. Can't run computers on TT, can run them on TAI.

[jxf]

Yes, I'm just saying that it's not accurate that "TAI doesn't change". It has notably and significantly changed a number of times.