Year Zero does not exist in the Christian Era, the Gregorian calendar or the Julian calendar. A year zero does exist in ISO 8601:2004 and in the astronomical year numbering with a defined year zero equal to 1 BC, as well as in some Buddhist and Hindu lunar calendars.
In QofDate, 1BC is immediately followed by 1AD and months are numbered from 1 to 12, not from zero.
However, locale-specific formats cannot be extended to the full range of QofDate dates because the locale data for these formats is only available to the underlying strftime implementation. The formats affected are those involving the E and O modifiers and other format specifiers that use the current locale. e.g. Japanese Emperor reigns, local numeric specifiers, translated days of the week / month etc. If these are used, only dates within the range of the locale-sensitive strftime on that platform can be supported (either inside or outside QofDate). This is not a restriction enforced by QOF, it is a gap in the libc API.
The full list of affected format specifiers is:
'%a', '%A', '%b', '%h', '%B', '%c', '%C', '%x', '%p', '%P', '%r', '%X', '%E' '%O'.
(Capital letter O, not zero: 0)
QofDate will attempt to fallback to a usable format if the date is out of range of the underlying strftime. e.g. QOF_DATE_FORMAT_UTC, QOF_DATE_FORMAT_UK, QOF_DATE_FORMAT_US, QOF_DATE_FORMAT_CE or QOF_DATE_FORMAT_ISO.
QOF_DATE_FORMAT_LOCALE and QOF_DATE_FORMAT_CUSTOM include locale-specific format specifiers.