A unit of time is any particular time interval, used as a standard way of measuring or expressing duration. The base unit of time in the International System of Units (SI), and by extension most of the Western world, is the second, defined as about 9 billion oscillations of the caesium atom. The exact modern SI definition is "[The second] is defined by taking the fixed numerical value of the cesium frequency, ΔνCs , the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be 9,192,631,770 when expressed in the unit Hz, which is equal to s−1." Historically, many units of time were defined by the movements of astronomical objects. These units do not have a consistent relationship with each other and require intercalation. For example, the year cannot be divided into twelve 28-day months since 12 times 28 is 336, well short of 365. The lunar month (as defined by the moon's rotation) is not 28 days but 28.3 days. The year, defined in the Gregorian calendar as 365.243 days has to be adjusted with leap days and leap seconds. Consequently, these units are now all defined for scientific purposes as multiples of seconds. Units of time based on orders of magnitude of the second include the nanosecond and the millisecond.

Historical

The natural units for timekeeping used by most historical societies are the day, the solar year and the lunation. Such calendars include the Sumerian, Egyptian, Chinese, Babylonian, ancient Athenian, Buddhist, Hindu, Islamic, Icelandic, Mayan, and French Republican calendars. The modern calendar has its origins in the Roman calendar, which evolved into the Julian calendar, and then the Gregorian calendar.

Scientific

Note: The light-year is not a unit of time, but a unit of length of about 9.5 petametres (9,454,254,955,488 km). Note: The parsec is not a unit of time, but a unit of length of about 30.9 trillion kilometres, despite movie references otherwise.

List

Interrelation

All of the formal units of time are scaled multiples of each other. The most common units are the second, defined in terms of an atomic process; the day, an integral multiple of seconds; and the year, usually 365 days. The other units used are multiples or divisions of these 3.