Editing Calendar date (section)

===Day and year only===
{{See also|calendar|time|date-time group|Japanese calendar|Wikibooks:English in Use/Time and Date}}

The U.S. military sometimes uses a system, known to them as the "Julian date format",<ref>{{Cite web |last=Hynes |first=John |title=A summary of time formats and standards |url=http://www.decimaltime.hynes.net/p/dates.html |access-date=2011-02-09 |website=www.decimaltime.hynes.net}}</ref> which indicates the year and the actual day out of the 365 days of the year (and thus a designation of the month would not be needed). For example, "11 December 1999" can be written in some contexts as "1999345" or "99345", for the 345th day of 1999.<ref name="km-iso">{{Cite web |title=International standard date and time notation |url=https://www.cl.cam.ac.uk/~mgk25/iso-time.html |access-date=2024-05-01 |website=www.cl.cam.ac.uk}}</ref> This system is most often used in US military logistics since it simplifies the process of calculating estimated shipping and arrival dates. For example: say a tank engine takes an estimated 35 days to ship by sea from the US to South Korea. If the engine is sent on 06104 (Friday, 14 April 2006), it should arrive on 06139 (Friday, 19 May). Outside of the US military and some US government agencies, including the [[Internal Revenue Service]], this format is usually referred to as "[[ordinal date]]", rather than "Julian date".<ref>Department of Defense. [https://web.archive.org/web/20110322220024/http://www.dtic.mil/whs/directives/corres/pdf/06-MAPAD_DEFINITIONS_TERMS_12-17-01.pdf "Definition of Terms."] March 11, 1997. Retrieved October 24, 2011.</ref>

Such ordinal date formats are also used by many computer programs (especially those for mainframe systems). Using a three-digit [[Julian date|Julian day number]] saves one byte of computer storage over a two-digit month plus two-digit day, for example, "January 17" is 017 in Julian versus 0117 in month-day format. [[OS/390]] or its successor, [[z/OS]], display dates in yy.ddd format for most operations.

[[UNIX time]] stores time as a number in seconds since the beginning of the UNIX Epoch (1970-01-01).

Another "ordinal" date system ("ordinal" in the sense of advancing in value by one as the date advances by one day) is in common use in astronomical calculations and referencing and uses the same name as this "logistics" system. The continuity of representation of period regardless of the time of year being considered is highly useful to both groups of specialists. The astronomers describe their system as also being a "[[Julian date]]" system.<ref>E. Kelly Taylor, ''America's Army and the Language of Grunts: Understanding the Army Lingo Legacy'' (Bloomington IN: AuthorHouse, 2009), 185. {{ISBN|1438962509}}, 9781438962504</ref>