Article 13c of the North American Stratigraphic Code: [...] Recommended (but not mandatory) abbreviations for such ages are SI (International System of Units) multipliers coupled with "a" for annum: ka, Ma, and Ga5 for kilo-annum (103 years), Mega-annum (106 years), and Giga-annum (109 years), respectively.
"Annum" is, of course, silly. It's the accusative; the nominative is _annus_.
[...] such qualifiers as "ago" or "before the present" are omitted after the value because measurement of the duration from the present to the past is implicit in the designation.
Makes sense for stratigraphers, not so much for evolutionary biologists...
Abbreviations for numbers of years, without reference to the present, are informal
I have no problem with calling this silly.