In article ,
Peter Bunclark writes:
The short answer is, no one knows by how much the earth will spin up or spin
down in the future (long before the heat death of the Universe, the Sun
will become a red giant, which is expected to have an impact on the
length of
the day). For a much more comprehensive answer, dig deep into
http://hpiers.obspm.fr
especially at the leap seconds section.

Pete.

PS, GMT is historic.

Day length is already increasing due to the transfer of angular momentum
to the moon via tides, and there will be a smaller exchange with the sun.
In principle you should be able to model this, but there are also
changes to the earth's moment of inertia which affect angular velocity,
and these seem to be quite unpredictable - who would have thought
before the fact that we wouldn't need leap seconds for the past
5 years, given the previous insertion rate?

J


[s.a.r. mod. note -- quoted text trimmed -- mjh]