- The kilogram just got a revamp. A time unit might be next. Updates to the metric measurement system went into force May 20, redefining the kilogram. The kilogram is now defined by a fundamental constant of nature, not a hunk of metal. Also redefined are the kelvin, the ampere, and the mole. https://www.sciencenews.org/article/kilogram-just-got-revamp-unit-time-might-be-next 14 comments science