UTC vs TDB
Versions of Celestia before v1.5.0 use UTC to calculate times and positions. Unfortunately, UTC includes "leap seconds" in order to stay aligned with the Earth's varying rotation. Leap seconds happen essentially randomly, when they are needed. Although Celestia does incorporate a table of leap second times, its use of UTC causes problems when used with ephemerides which are defined using TDB. Starting with v1.5.0, although it still displays UTC on the screen, Celestia uses the TDB time scale internally for everything else. As a result, Celestia places objects much more accurately than before.
It may be helpful for Celestia users to understand the various time scales that are used in the field of astronomy. The most familiar is local time, which is probably what you see if you look at a nearby clock. The local time zone determines the difference between local time and Coordinated Universal Time, or UTC. UTC is the current time at the zero degree meridian; it replaces Greenwich Mean Time as the standard world clock.
Astronomical ephemerides are typically defined in terms of Barycentric Dynamical Time, or TDB. TDB is the time measured by a clock at the solar system barycenter. It differs from Terrestrial Time (TT)--the time measured by a clock on the Earth--because of relativistic effects, but the difference between the two scales is always less than 0.002 seconds. The two scales can usually be considered equivalent in Celestia.
TT is ahead of International Atomic Time (TAI) by constant value of 32.184 seconds. Finally, UTC differs from TAI by some integer number of leap seconds. Leap seconds are inserted occasionally because the Earth's rotation is irregular, and it's desirable to keep our everyday time scale from drifting with respect to the terrestrial day-night cycle--except at the poles, the sun should be in the sky at noon. The most recent leap second occurred at 23:59:60 on December 31, 2008 and made UTC 34 seconds behind TAI. (See Wikipedia on TAI)
To briefly summarize the relationship between time scales:
TT = TAI + 32.184 TAI = UTC + leap second count Thus, TT = UTC + 32.184 + leap second count
UTC is used in the Celestia's Set Time dialog and it's also the time displayed in the upper right of the screen. Unless you're creating scripts or add-ons for Celestia, UTC is all that you will see. But everywhere else, the time scale is TDB: xyz trajectory files, attitude files, beginning, ending, and epoch times in .ssc files.