See the FAQ of the JPL's website:
Coordinate time is the time used in the development of ephemerides
for solar system objects. Under General Relativity, the rate at which
actual clocks tick is called proper time. The rate of proper time
depends on the location and motion of the clock, so there is no single
proper time for the solar system as a whole. So for representing the
motions of he solar system bodies using numerical integration, a time
scale called 'coordinate time' is used, which is not the rate of any
physical clock, but a parameter for which the equations of motion are
simply expressed.
The integrated ephemerides for solar system bodies are stored as
tables of positions and velocities as a function of coordinate time.
In order to evaluate a measurement, such as the round-trip light time
from Earth to Mars, or the direction to Saturn as seen from Earth, the
proper times of the measurement must be converted to coordinate time
before looking up the positions.
Currently the International Astronomical Union (IAU) has defined two
coordinate times for the solar system and both are equally accurate.
The coordinate time used for the JPL ephemerides, as the independent
variable in the solar system, barycentric relativistic, equations of
motion, is Barycentric Dynamical Time (TDB) or, in French, Temps
Dynamique Barycentrique. This coordinate time is defined such that in
the vicinity of the Earth the difference in coordinate time and
international atomic time (TAI) is 32.184 seconds plus a small
variation that is less than 3 milliseconds. TAI differs from Universal
Coordinate Time (UTC) by an integer number of seconds (34 as of July
1, 2012), which changes only when leap seconds are added. UTC is the
basis for civil time (e.g., Pacific Standard Time equals UTC - 8
hours).
So the time is neither UTC nor GMT, it is TDB to avoid the problems with leap seconds.
Many thanks to Christiano for finding my error about TAI.
