Timescales
In Astronomy, two types of time scales are used: those obtained from very accurate clocks, and those based on the orientation of the Earth. We will explain why both classes are needed and how to try to combine them.
Comparisons between hundreds of atomic clocks allow to obtain a very uniform time scale: International Atomic Time (TAI), based on the second of the International System of Units (IS). In Astronomy, for reasons of continuity with previous scales the Terrestrial Time (TT) is often used instead of TAI. TT differs in a fixed amount (32.184 seconds) from TAI, but is also based on the SI second, so we can get one from the other by simple addition or subtraction. All time scales obtained from precise clocks are conceptually based on General Relativity's terrestrial proper time, and can be mapped to the time coordinates of the physical models of our Solar System or our Galaxy. Essentially, the idea is that the TT/TAI is associated to the 'time' in the equations of Physics.
On the other hand, daily activities have traditionally used time scales based on the rotation of the Earth. Concepts such as noon or midnight are obviously linked to the orientation of the Earth to the Sun. Based on that idea, we currently have a family of time scales called Universal Time (UT), which replaces the old Greenwich Mean Time (GMT). UT is defined by the average apparent motion of the Sun (which is the result of the Earth's rotation, combined with the motion of the Earth around the Sun.) Depending on how we define such average, we have several versions of UT (like UT0, UT1 or UTC), although unless you need a precision better than 1 second one can speak simply of "UT". So, while scales like the TAI are obtained from atomic clocks, the UT is determined from astronomical and geodetic observations.
Since the rotation of our planet is irregular (the Earth is not a clock), Universal Time is shifted with respect to TT and TAI. This drift is caused by several factors, including tides. At present, the difference between TT and UT is just over a minute. To reconcile our natural times (UT) with the highly uniform time scales such as the TAI, a variant of UT called Coordinated Universal Time (UTC) was defined, based (like the TAI) on the IS second, but specified as not departing from UT1 by more than 0.9 seconds. To achieve this, a correction of one second is applied to UTC when appropriate. For example, such corrections were applied in December 2005 and December 2009.
The official times of the various countries are based on UTC. By applying the appropriate timezone, if we have a time expressed in UTC we can easily get the official (legal) time. For example, in the case of Hawaii we need to subtract ten hours from UTC to get the official time, while in London the UTC gives the official time during winter time, and differs in one hour (DST) during the summer.
Why not always use UT/UTC in astronomical ephemerides instead of TT? The difference between UT/UTC and TT/TAI cannot be predicted with enough accuracy, not to talk about its long-term evolution. When no high accuracy is required, the ephemerides relating to the present day generally use UTC to give the times; however, for the (more or less) remote future or the past it is necessary to use TT, as we cannot know the UT-TT difference with enough precision, and the uncertainty grows as we depart from the current epoch.