Time units: from Planck time to Hubble time

Why these strange time units exist

A second is excellent for ordinary clocks, but science often needs scales that are far outside ordinary perception. A nuclear physicist can say a chain-reaction step takes a shake instead of repeatedly writing 10 nanoseconds. A chemist or physicist can use a jiffy for the time light crosses a nuclear length scale. At the extreme, Planck time is not a practical measured interval; it is a natural scale made from fundamental constants, useful when gravity and quantum mechanics collide.

Human language kept other units because institutions kept using them. Lustrum survived from the Roman census and purification ceremony into English as a five-year span. Score survived as a tally word for twenty, made famous to many readers by Lincoln's "four score and seven years ago." Fortnight is older still: literally fourteen nights. It remains normal in British and some Commonwealth English, but American English usually says "two weeks," so the word sounds increasingly literary or imported to U.S. readers.

The table keeps these mixed traditions honest by reducing each unit to seconds and linking its source. SI prefixes come from BIPM; CODATA constants come from NIST; geological Ma and Ga follow GSA time-scale usage; cultural terms cite dictionaries or primary historical texts.

Sources include BIPM SI prefixes, NIST unit factors, and 30 per-row citations in the converter.