Byr
（重定向自Gigayear）
A gigaannum or gigayear (Byr, Gyr or Gy) is a period of one billion years (10 or 1,000,000,000 years). Retrospect time periods may be expressed as X billion years ago (bya). The prefix giga- is preferred to billion- to avoid confusion in the long and short scales over the meaning of billion; the postfix annum may be further qualified for precision as a sidereal year or Julian year.