Editing List of non-standard dates (section)

===February 30===
{{see also|Swedish calendar}}
[[File:feb1712.jpg|thumb|right|300px|Swedish calendar for February 1712]]

'''February 30''' is a day that does not occur on the [[Gregorian calendar]], where the month of [[February]] contains only 28 days, or 29 days in a [[leap year]]. However, from a historical perspective February 30 has been used at least once and appears in some [[#Reform calendars|reform calendars]].

The thirteenth-century scholar [[Johannes de Sacrobosco]] claimed that in the [[Julian calendar#Sacrobosco's incorrect theory on month lengths|Julian calendar]], [[February]] had 30 days in leap years from 45 BC until 8 BC, when [[Caesar Augustus|Augustus]] allegedly shortened [[February]] by one day to give that day to the month of [[August]] named after him so that it had the same length as the month of [[July]] named after his adoptive father, [[Julius Caesar]]. However, all historical evidence refutes [[Johannes de Sacrobosco|Sacrobosco]], including dual dates with the [[Coptic calendar|Alexandrian calendar]].<ref>Roscoe Lamont, "[http://articles.adsabs.harvard.edu//full/1919PA.....27..579P/0000583.000.html The Roman calendar and its reformation by Julius Caesar]", ''[[Popular Astronomy (US magazine)|Popular Astronomy]]'' '''27''' (1919) 583–595. Sacrobosco's theory is discussed on pages 585–587.</ref>

February 30 was a day that happened in [[Sweden]] in 1712.<ref name = timeanddate>{{cite web
 |url=http://www.timeanddate.com/date/february-30.html
 |title=February 30 Was a Real Date
 |publisher=timeanddate.com
 |access-date=2024-07-11
 |first=Vigdis
 |last=Hocken
 |url-status=live
 |archive-url=https://web.archive.org/web/20240329043659/https://www.timeanddate.com/date/february-30.html
 |archive-date=2024-03-29
}}</ref> This occurred because, instead of changing from the [[Julian calendar]] to the [[Gregorian calendar]] by omitting a block of consecutive days, as had been done in other countries, the [[Swedish Empire]] planned to change gradually by omitting all [[leap day]]s from 1700 to 1740, inclusive, so that the next leap year after 1696 would not be until 1744. Although the leap day was omitted in February 1700, the [[Great Northern War]] began later that year, diverting the attention of the Swedes from their calendar so that they did not omit leap days on the next two occasions; 1704 and 1708 remained leap years.<ref name="Bauer">{{cite book|last1=Bauer|first1=R. W.|title=Calender for Aarene fra 601 til 2200|date=1868|location=Copenhagen, Denmark|publisher=Dansk Historisk Fællesråd (1993 reprint)|isbn=87-7423-083-2|page=100}}</ref>

To avoid confusion and further mistakes, the [[Julian calendar]] was restored in 1712 by adding a second leap day, thus giving that year the only known actual use of February 30 in a calendar. That day corresponded to February 29 in the [[Julian calendar]] and to March 11 in the [[Gregorian calendar]].<ref name="Bauer"/><ref>{{cite book|last1=Vallerius|first1=Johannes|title=Allmanach på åhret effter Christi födelse 1712|date=1711|location=Lund, Sweden}}</ref> The Swedish conversion to the [[Gregorian calendar]] was finally accomplished in 1753, when February 17 was followed by March 1.<ref name="Bauer"/>

Artificial calendars may also have 30 days in [[February]]. For example, in a climate model the statistics may be simplified by having 12 months of 30 days. The [[Hadley Centre]] General Circulation Model is an example.<ref>{{cite web|url=http://www.metoffice.com/research/hadleycentre/models/GDT/ch23.html|title=Hadley Centre: GDT netCDF conventions|date=November 22, 2005|work=MetOffice.com|access-date=March 21, 2017|url-status=dead|archive-url=https://web.archive.org/web/20051122015007/http://www.metoffice.com/research/hadleycentre/models/GDT/ch23.html|archive-date=November 22, 2005}}</ref>