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Undecimber or Undecember is a name for a thirteenth month in a calendar that normally has twelve months. Duodecimber or Duodecember is similarly a fourteenth month.


The word undecimber is based on the Latin word undecim meaning "eleven". It is formed in analogy with December, which, though the twelfth month in the Gregorian calendar, derives from decem meaning "ten". The word undecember (abbreviated Vnde) is recorded from a Roman inscription according to the Oxford Latin Dictionary, which defines it as "a humorous name given to the month following December".[1]

Some recent authors report the names "Undecember" and "Duodecember" for the two intercalary months inserted between November and December upon the adoption of the Julian calendar in 44 BC, including the World Calendar Association[2] and Isaac Asimov.[3] This claim has no contemporary evidence; Cicero refers to the months as intercalaris prior and intercalaris posterior in his letters.[4]

Historian Cassius Dio tells that Licinus, procurator of Gaul, added two months to the year 15 BC, because taxes were paid by the month.[5] Though not named by Dio, who wrote in Greek, August Immanuel Bekker suggested these might have been called "Undecember" and "Duodecember".[citation needed]


In the Java Platform, Standard Edition, the java.util.Calendar class includes support for calendars which permit thirteen months.[6] Although the Gregorian calendar used in most parts of the world includes only twelve months, there exist some lunar calendars that are divided into synodic months, with an intercalary or "leap" month added in some years. For example, in the Hebrew calendar seven years out of every nineteen (37%) have the "embolismic month" Adar I.[6][7] The constant java.util.Calendar.UNDECIMBER represents such a month.[8]

Accounting (called "Month 13")[edit]

In accounting, a thirteenth month is sometimes used to adjust financial statements for an entire year without affecting monthly results. For example, an organization may wish to adjust its books to reflect the fact that some of its sales and resulting payments due from customers will not be paid. If an organization only does this once per year the organization can attribute these adjustments to "Month 13" so as not to inaccurately post a full year's worth of write offs to one month.[9]

See also[edit]


  1. ^ Glare, P.G. (2002). Oxford Latin Dictionary. Clarendon Press. ISBN 0-19-864224-5.
  2. ^ Jézéquel, Jules (1937). "Why the World Needs This Reform". Journal of calendar reform. New York City: World Calendar Association. 7: 64.
  3. ^ Asimov, Isaac; John Bradford (1963). The clock we live on (revised ed.). Collier Books. p. 118. ISBN 0-200-71100-8.
  4. ^ Heitland, W.E. (1909). "Chap LVIII: From the Battle of Thapsus to the death of Caesar: 46–44 B.C.". The Roman Republic. Vol.3. Cambridge: Cambridge University Press. pp. 347 §1269. ISBN 0-89005-577-7. |volume= has extra text (help)
  5. ^ Cassius Dio (1914–1927). "LIV 21.5". Roman History. Loeb Classical Library. Translated by Earnest Cary. Harvard University Press. p. 335. ISBN 0-665-72855-7.
  6. ^ a b Janert, Philipp K. (2007-06-04). "Making Sense of Java's Dates". On Java. O'Reilly Media. Retrieved 2007-12-13.
  7. ^ "In a leap year, is the extra month Adar I or Adar II?". Mi Yodeya StackExchange.
  8. ^ "java.util Class Calendar: UNDECIMBER". Java Platform, Standard Edition 6: API Specification. Sun Microsystems. 2008. Retrieved 2009-05-22.
  9. ^ L. Evans, Denise L.; Evans, William (2007). The Complete Real Estate Encyclopedia. The McGraw-Hill Companies, Inc. Retrieved 18 December 2020.

External links[edit]