The abbreviation myr, "million years", is a unit of a quantity of years, or 31.536 teraseconds.


Myr is in common use where the term is often written, such as in Earth science and cosmology. Myr is also used as with MYA. Together they make a reference system, one to a quantity, the other to a particular place in a year numbering system that is time before the present.
Myr is deprecated in geology, but in astronomy myr is standard.
Where "myr" is seen in geology it is usually "Myr". In astronomy it is usually "Myr".


In geology the debate of the millennia concerns the use of myr remains open concerning "the use of Myr plus Mya" versus "using Mya only". In either case the term Ma is used in geology literature conforming to ISO 31-1 and NIST 811 recommended practices. Traditional style geology literature is written
The "ago" is implied, so that any such year number "X Ma" between 66 and 145 is "Cretaceous", for good reason. But the counter argument is that having myr for a duration and Mya for an age mixes unit systems, and tempts capitalization errors: "million" need not be capitalized, but "mega" must be; "ma" would technically imply a milliyear. On this side of the debate, one avoids myr and simply adds ago explicitly, as in
In this case, "79 Ma" means only a quantity of 79 million years, without the meaning of "79 million years ago".