calendarDuration represents a period of time in variable-length calendar
components. For example, years and months can have varying numbers of days, and days
in time zones with Daylight Saving Time have varying numbers of hours. A
calendarDuration does arithmetic with "whole" calendar periods.
durations cannot be directly combined, because
they are not semantically equivalent. (This may be relaxed in the future to allow
durations to be interpreted as numbers of days when combined with
d = datetime('2011-03-04 00:00:00') ⇒ 04-Mar-2011 cdur = calendarDuration(1, 3, 0) ⇒ 1y 3mo d2 = d + cdur ⇒ 04-Jun-2012