Myr is in common use in fields such as Earth science and cosmology.
In geology, a debate remains open concerning the use of Myr (duration) plus Mya (million years ago) versus using only the term Ma.
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: The Cretaceous started 145 Ma ago and ended 66 Ma ago, lasting for 79 Ma.In this case, "79 Ma" means only a quantity of 79 million years, without the meaning of "79 million years ago".