Undecimber

Undecember is a name for the 13th month in a calendar that normally has 12 months.

Etymology
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".

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 BCE, including the World Calendar Association and Isaac Asimov. This claim has no contemporary evidence; Cicero refers to the months as "intercalates prior and intercalates posterior" in his letters.

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

Computing
In the Java Platform, Standard Edition, the  class includes support for calendars which permit thirteen months. 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. The constant  represents such a month.

Accounting (called "Month 13")
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.

Further months
Similar names exist for additional months in calendars extended beyond 13 months. For instance, duodecember is a 14th month, and tridecember is a 15th month.