DateTime::Set is a module for datetime sets.  It can be used to handle
two different types of sets:
 * Fixed sets of predefined datetime objects.  For example, if we wanted
   to create a set of datetimes containing the birthdays of people in
   our family.
 * Recurring sets, such as "every Wednesday", or "noon on the 15th day
   of every month".  This type of set can have fixed starting and ending
   datetimes, but neither is required.
