Bya

BYA or B.Y.A. is an abbreviation for "Billion Years Ago". It is used as a unit of time to denote length of time before the present, in 109 years. This initialism is often used in the sciences of astronomy, geology, and paleontology. The "billion" in bya is the 109 "billion" of the short scale of the United States, not the long-scale 1012 "billion" of some European usage. Billion by this convention (109) is often called a "thousand million" in the UK and a "milliard" in some other countries. For this reason, there is potential for some confusion, and some scientists prefer the unit Gya, while others prefer Ga (Giga-annum); however, bya remains in more widespread use. In 1974, the UK switched from the long scale to the short scale. Related units are mya ("million years ago"), and byr ("billion years"), which are traditionally written uncapitalized, while Ga or Gya starts with a capital letter.