The abbreviation myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.6 teraseconds. 

Myr is in common use where the term is often written, such as in Earth science and cosmology. Myr is seen with "mya", "million years ago". 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" (a unit of mega-years). In astronomy it is usually "MYR" (million years). 

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 (now ISO 80000-3) 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" (a thousandth of a year, or 8 hours). On this side of the debate, one avoids "myr" and simply adds "ago" explicitly (or adds "BP"), as in In this case, "79 Ma" means only a quantity of 79 million years, without the meaning of "79 million years ago".
how do they do that?
On this side of the debate, one avoids "myr" and simply adds "ago" explicitly (or adds "BP")