date-time-ywd-before-year-0

Arguments: date-time

Returns the value of the ymd-yd-before-year-0 of date-time, which must be a date-time object. The value can be t or nil. t means the date in date-time is definitely before the year 0. nil can mean that the date is not before the year 0 or that the slot value has not been calculated and holds its initial value (which is nil).

The value depends on the value of the ywd-century (reader date-time-ywd-century: if that value is positive, the century is not before year 0 and so nil is the proper value for this function; linewise the century being negative means t is the correct value for this function. If the value of the ymd-yd-century is nil, the correct value for this function cannot be determined and the initial value (nil) is returned.

date-time should be complete (see complete-date-time) before calling this function. Otherwise the slot may be unset even though there is enough date information to set it accurately.

This is not a particular useful value for user code (its is useful for internal code). Users may wish to just look at the century value to determine whether the date is before year 0 or not, if that information is knowable. See also date-time-ymd-yd-before-year-0, which give a different value on January 1, year 0.

See date-time.htm for information on support in Allegro CL for parsing and generating time expressions using the ISO 8601 standard.

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Created 2019.8.20.