Atomic Time
Atomic time is the most precise measurement of time ever created, based as it is on the atomic properties of caesium clocks around the world.The full name for atomic time is International Atomic Time or Temps Atomique International (TAI). The official definition of a "second" is based on the transition between energy states of the Caesium-133 atom. Atomic clocks measure this transition. To obtain even greater accuracy, TAI is determined not just by one atomic clock but from a number of different clocks based around the world.
TAI was defined to "begin" in 1958 with the atomic time equal to UT1 ("solar time") on January 1st 1958.
UTC
In 1972 TAI was used as the basis of Coordinated Universal Time (UTC), the worldwide time standard. Atomic clock data from around the world is fed into the International Bureau of Weights and Measures (BIPM) in Paris which maintains official UTC.Unfortunately, atomic time is in some ways too accurate!
The earth itself is not as regular as the caesium atom. As a result, if left uncorrected atomic time and "real" solar time would slowly drift apart. For this reason UTC is adjusted by adding or subtracting leap seconds as required.