DateTime::Set is a module for date/time sets. It can
be used to handle two different types of sets.
The first is a fixed set of predefined datetime objects.
For example, if we wanted to create a set of dates
containing the birthdays of people in our family.
The second type of set that it can handle is one based
on the idea of a recurrence, 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. So our "every Wednesday set" could
be "every Wednesday from the beginning of time until
the end of time", or "every Wednesday after 2003-03-05
until the end of time", or "every Wednesday between
2003-03-05 and 2004-01-07".
