Point types are usually assumed to have a first (minimum) and last (maximum) value. This appendix concerns itself with point types such as days of the week for which that assumption isn’t necessarily valid. For example, in the case of days of the week, an interval such as “Friday to Tuesday” does make sense, at least intuitively. Allen’s operators are reexamined in this light and are found still to be applicable, though their definitions need to be refined somewhat. The same goes for the interval UNION, INTERSECT, and MINUS operators, also for the PACK and UNPACK operators.
Keywords: cyclic type; periodic type; wraparound interval; Allen’s operators
What goes around comes around.
—late 20th century catchphrase