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".