Oh. In UT1 there number of seconds in a day is constant, so it's value is defined by the rotation of the earth which is slowing. UTC is better than UT1 in that

Oh. In UT1 there number of seconds in a day is constant, so it's value is defined by the rotation of the earth which is slowing. UTC is better than UT1 in that regard. Maybe UT1 is fine for humans, but really humans just assume ut1.