The period facet defines the
frequency of recurrence (after what duration it recurs) for time types.
Its value is a time duration.
The duration facet defines the
the length of the duration for time types.
Its value is also a time duration.
Applies to time types:
time, timeInstant, timePeriod,
date, month, year,
recurringDay, and recurringDate.
For example, you might use the period facet to define a
twoWeek type with a fourteen day
and a fourteen day period
duration for paychecks.