Norman E. Holden, Mauro L. Bonardi, Paul De Bièvre, Paul R. Renne, and Igor M. Villa (2011). «IUPAC-IUGS common definition and convention on the use of the year as a derived unit of time (IUPAC Recommendations 2011)». Pure and Applied Chemistry83 (5): 1159–1162. doi:10.1351/PAC-REC-09-01-22.
Russ Rowlett. «Units: A». How Many? A Dictionary of Units of Measurement. University of North Carolina. Αρχειοθετήθηκε από το πρωτότυπο στις 20 Δεκεμβρίου 2008. Ανακτήθηκε στις 9 Ιανουαρίου 2009.
North American Commission on Stratigraphic Nomenclature. «Article 13 (c)». North American Stratigraphic Code. Αρχειοθετήθηκε από το πρωτότυπο στις 2014-02-02. https://web.archive.org/web/20140202155725/http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#Article13. Ανακτήθηκε στις 2012-10-29. «(c) Convention and abbreviations. – The age of a stratigraphic unit or the time of a geologic event, as commonly determined by numerical dating or by reference to a calibrated time-scale, may be expressed in years before the present. The unit of time is the modern year as presently recognized worldwide. Recommended (but not mandatory) abbreviations for such ages are SI (International System of Units) multipliers coupled with "a" for annum: ka, Ma, and Ga for kilo-annum (103 years), Mega-annum (106 years), and Giga-annum (109 years), respectively. Use of these terms after the age value follows the convention established in the field of C-14 dating. The "present" refers to 1950 AD, and 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. In contrast, the duration of a remote interval of geologic time, as a number of years, should not be expressed by the same symbols. Abbreviations for numbers of years, without reference to the present, are informal (e.g., y or yr for years; my, m.y., or m.yr. for millions of years; and so forth, as preference dictates). For example, boundaries of the Late Cretaceous Epoch currently are calibrated at 63 Ma and 96 Ma, but the interval of time represented by this epoch is 33 m.y.».
Russ Rowlett. «Units: A». How Many? A Dictionary of Units of Measurement. University of North Carolina. Αρχειοθετήθηκε από το πρωτότυπο στις 20 Δεκεμβρίου 2008. Ανακτήθηκε στις 9 Ιανουαρίου 2009.
North American Commission on Stratigraphic Nomenclature. «Article 13 (c)». North American Stratigraphic Code. Αρχειοθετήθηκε από το πρωτότυπο στις 2014-02-02. https://web.archive.org/web/20140202155725/http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#Article13. Ανακτήθηκε στις 2012-10-29. «(c) Convention and abbreviations. – The age of a stratigraphic unit or the time of a geologic event, as commonly determined by numerical dating or by reference to a calibrated time-scale, may be expressed in years before the present. The unit of time is the modern year as presently recognized worldwide. Recommended (but not mandatory) abbreviations for such ages are SI (International System of Units) multipliers coupled with "a" for annum: ka, Ma, and Ga for kilo-annum (103 years), Mega-annum (106 years), and Giga-annum (109 years), respectively. Use of these terms after the age value follows the convention established in the field of C-14 dating. The "present" refers to 1950 AD, and 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. In contrast, the duration of a remote interval of geologic time, as a number of years, should not be expressed by the same symbols. Abbreviations for numbers of years, without reference to the present, are informal (e.g., y or yr for years; my, m.y., or m.yr. for millions of years; and so forth, as preference dictates). For example, boundaries of the Late Cretaceous Epoch currently are calibrated at 63 Ma and 96 Ma, but the interval of time represented by this epoch is 33 m.y.».
«Time Units». Geological Society of America. Αρχειοθετήθηκε από το πρωτότυπο στις 16 Ιουνίου 2016. Ανακτήθηκε στις 17 Φεβρουαρίου 2010.
