Two of the units of time used to describe these very ancient events are the Ga (or Gya) and the Ma (or Mya).
The Ga is the abbreviation for Giga annum is 1×10 raised to the 9th power (or a billion) or Gya for Giga years ago. Alternatively the Ma (or Mya) is the abbreviation for millions of years ago; so 4.6 Ga (billion years ago) = 4600 Ma (or million years ago).
