Editing Year (section)

=== Abbreviations for "years ago" ===
{{anchor|Abbreviations yr and ya}}{{anchor|years ago}}{{anchor|kya}}{{anchor|Mya}}{{anchor|Bya}}
{{further|Before Present}}

In geology and paleontology, a distinction sometimes is made between abbreviation "yr" for ''years'' and "ya" for ''years ago'', combined with prefixes for thousand, million, or billion.<ref name="AGUStyle" /><ref name="NASC">
{{cite web
 |author=North American Commission on Stratigraphic Nomenclature|url=http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#Article13
 |title=North American Stratigraphic Code (Article 13 (c))
 |quote=(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 annus: ka, Ma, and Ga for kilo-annus (10<sup>3</sup> years), Mega-annus (10<sup>6</sup> years), and Giga-annus (10<sup>9</sup> 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 AD 1950, 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.
}}</ref> In archaeology, dealing with more recent periods, normally expressed dates, e.g. "10,000 BC", may be used as a more traditional form than [[Before Present]] ("BP").

These abbreviations include:
{|class="wikitable"
! rowspan=2|{{Anchor|mya}}Non-SI<br />abbreviations
! rowspan=2|Short for
! rowspan=2|SI-prefixed<br />equivalent
! rowspan=2|Definition
! colspan=2|Examples
|-
!Event
!Time
|-
| {{center|[[kyr]]}} || kilo years || ka ||Thousand years||colspan=2|
|-
| {{center|[[myr]]<br />Myr}} || million years<br />Mega years || Ma ||Million years||colspan=2|
|-
| {{center|[[billion years|byr]]<br />Gyr}} || billion years<br />Giga years || Ga ||Billion years<br />([[Long and short scales|thousand million]] years)||colspan=2|
|-
| {{center|kya}} || kilo years ago || || Time ago in ka ||
* Appearance of ''[[Homo sapiens]]''
* [[Out-of-Africa migration]]
* [[Last Glacial Maximum]]
* [[Neolithic Revolution]]
||
Around 200 kya<br />Around 60 kya<br />Around 20 kya<br />Around 10 kya
|-
| {{center|mya<br />Mya}}|| million years ago<br />Mega years ago || ||Time ago in Ma ||
* [[Pliocene]]
* [[Brunhes–Matuyama reversal|Last geomagnetic reversal]]<ref>
{{cite journal
 |first=Bradford M.
 |last=Clement
 |title=Dependence of the duration of geomagnetic polarity reversals on site latitude
 |date=April 8, 2004
 |journal=Nature
 |volume=428
 |pmid=15071591
 |doi=10.1038/nature02459
 |issue=6983
 |pages=637–640
 |bibcode = 2004Natur.428..637C
 |s2cid=4356044
}}</ref>
* The ([[Eemian Stage]]) [[Last Glacial Period]] started
* The [[Holocene]] started
| 5.3 to 2.6 mya<br />0.78 mya<br />0.13 mya<br />0.01 mya
|-
| {{center|bya<br />Gya}} || billion years ago<br/>giga years ago|| ||Time ago in Ga ||
* Oldest [[Eukaryote]]s
* Formation of the [[Earth]]
* [[Big Bang]]
|2 bya<br />4.5 bya<br />13.8 bya
|}

Use of "mya" and "bya" is deprecated in modern geophysics, the recommended usage being "Ma" and "Ga" for dates [[Before Present]], but "m.y." for the durations of epochs.<ref name="AGUStyle" /><ref name="Strat" /> This ''ad hoc'' distinction between "absolute" time and time intervals is somewhat controversial amongst members of the Geological Society of America.<ref>{{cite web|publisher=Geological Society of America|title=Time Units|url=https://www.geosociety.org/TimeUnits/|access-date=February 17, 2010| archive-date=June 16, 2016|archive-url=https://web.archive.org/web/20160616100504/https://www.geosociety.org/TimeUnits/}}</ref>