<http://en.wikipedia.org/wiki/International_Atomic_Time>:

  In the 1970s, it became clear that the clocks participating in TAI
  were ticking at different rates due to gravitational time dilation,
  and the combined TAI scale therefore corresponded to an average of
  the altitudes of the various clocks.  Starting from Julian Date
  2443144.5 (1 January 1977 00:00:00), corrections were applied to the
  output of all participating clocks, so that TAI would correspond to
  proper time at mean sea level (the geoid).  Because the clocks had
  been on average well above sea level, this meant that TAI slowed
  down, by about 10e−12.  The former uncorrected time scale continues
  to be published, under the name EAL (Echelle Atomique Libre, meaning
  Free Atomic Scale).

In our relativistic universe the quest for simultaneity is hopeless.
High-precision clocks are common items on spacecraft.  Scheduling
burns, data-collection, and transmissions must be done across
different frames of reference.

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