Slots on this function:
The publication date of a document. If the document isn't
formally published, e.g., a painting, then it is the year of creation.
The date is a timepoint for which at least the year is known.
In some cases, the month and day are also known.
- Arity: 2
- Domain: Document
- Range: Calendar-year
- Calendar-date is a subclass-of calendar-year, because it
requires the same constraint (that a year be known) plus
a few more (that a month and day be known). This means
that if we require publication dates to be calendar-years,
then we also allow more precise timepoint specifications
as well. Thus, documents for which a month field is needed
can get the month from the publication date.