Year zero
ISO 8601:2004 (and previously ISO 8601:2000, but not ISO 8601:1988) explicitly uses astronomical year numbering in its date reference systems. Because it also specifies the use of the proleptic Gregorian calendar for all years before 1582, some readers incorrectly assume that a year zero is also included in that proleptic calendar, but it is not used with the BC/AD era. The "basic" format for year 0 is the four-digit form 0000, which equals the historical year 1 BC. Several "expanded" formats are possible: −0000 and +0000, as well as five- and six-digit versions. Earlier years are also negative four-, five- or six-digit years, which have an absolute value one less than the equivalent BC year, hence -0001 = 2 BC. Because only ISO 646 (7-bit ASCII) characters are allowed by ISO 8601, the minus sign is represented by a hyphen-minus.
Year Zero (political notion)
The term Year Zero ( "chhnam saun"), applied to the takeover of Cambodia in April 1975 by the Khmer Rouge, is an analogy to the Year One of the French Revolutionary Calendar. During the French Revolution, after the abolition of the French monarchy (September 20, 1792), the National Convention instituted a new calendar and declared the beginning of the Year I. The Khmer Rouge takeover of Phnom Penh was rapidly followed by a series of drastic revolutionary de-industrialization policies resulting in a death toll that vastly exceeded that of the French Reign of Terror.