Universal Time (UT) is not a measure of physical time, but rather is a
measure of the rotation angle of the Earth with respect to distant
quasars. A UT second is identified with a fixed amount of rotation of
the Earth, which equals 1/86400 of a mean solar day. That's why every
day has 86400 UT seconds.
Quite right. Buit the whole point of UTC is that its seconds are not angles,
but SI = TAI seconds. There are a variable number of these in a day, and
a UTC clock will indeed report 23:59:60 at the end of a day with a leap
second in it (and other civil-time clocks will similarly report :60 in whatever
hour, according to their timezone offsets). See
Now there are indeed exactly 86400 _Posix_ seconds in a day, which is
achieved by giving two seconds the same label if it is a leap day. But
that has nothing to do with either TAI or UTC.
UTC is kept within 0.9 seconds of UT1 (a
version of UT with certain corrections applied), so over long time
periods, with the leap seconds taken into account, UTC seconds are equal
to UT seconds.
No, in the long run UTC time is equal to UT1 time. That's not the same thing at all.
I am expressing my opinion. When my honorable and gallant friend is
called, he will express his opinion. This is the process which we
call Debate. --Winston Churchill