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.
Date and time notation in Serbia
Years can be written either with four or two digits, and in the latter case, century is usually replaced with an apostrophe (31. 12. ’06.). Leading zero is rarely used, and in those cases which are considered bad practice, only with months (however it is also optional for the days) (6. 05. 2006.). When literal names of the months are used they are not capitalized, and the four-digit format for the year is always used (31. December 2006). Yet another alternative is to use Roman numerals to indicate the month. In this case a dot is omitted (31. XII 2006).