The International System of Units (SI) identifies "second" as the base unit for time. It is determined using radiation periods for the cesium 133 atom.
Development of international units started in the late 18th century. Each SI unit has been established in a distinct, consistent and often complicated way. The international unit of length is the meter, and mass is universally measured in kilograms. Electric currents are measured in amperes, and thermodynamic temperature is gauged using the unit kelvin. Moles are is used to calculate amounts of a substance, and the luminous intensity of a substance is measured in candela.