The time-scale used for all precise timekeeping, including civil time. It is based upon atomic frequencies and is the most accurate and consistent available today. The fundamental unit is the SI second, which is defined in terms of a particular spectral line of the caesium-133 atom. The frequency of this microwave line is adopted as precisely 9 192 631 770 Hz.

The SI second is the basis for International Atomic Time (TAI). TAI was officially introduced by international agreement in January 1972, but had been available since 1955. When TAI was officially introduced, the length of the SI second was the same as that of a second in Ephemeris Time, the time-scale then used in astronomy. The two time-scales differed by a fixed amount (ET = TAI + 32.184s). Strictly speaking, however, the two definitions of the second were conceptually different. This difference was removed in 1984 when Terrestrial Dynamical Time (now known simply as Terrestrial Time) was introduced to replace Ephemeris Time. Terrestrial Time has the SI second as its basic unit and differs from TAI only by the constant offset mentioned above.

TAI is the basis not just of astronomical time-scales but also of civil timekeeping. Broadcast time signals use Coordinated Universal Time (UTC), which is TAI with an offset of a whole number of seconds. This offset requires occasional adjustment by the insertion of leap seconds.